Simple answer: Loyalty.
To the Bugle itself, of course, yes. But also... personal loyalty specifically to it's publisher, J. Jonah Jameson.
I know it may not seem as such on page, but believe it or not, Peter and JJ have a very long, very complicated, but also very deep, close relationship with each other. This goes beyond their long-standing Boss/Employee interaction at the Bugle, and into a kind of "Grumpy Uncle/ Well meaning Nephew" sort of vibe. While Jonah has taken personal issue with Spider-Man for years, and has busted Peter's chops on more than one occasion, Jameson has also been one of Peter's most staunch supporters and male influences on a personal level.
Let me elaborate...
Given Marvel's sliding timeline in "our" 616 universe, Peter Parker is now in either his late 20s or early 30s. He and Jonah have known each other all those years, seeing each other almost daily during that time. Jonah has often (out of ear shot, or begrudgingly) called Peter his "best photographer." In most iterations, Peter started freelancing for Jonah when he was only 15....
Now this might be something of a minor ret-con, but it does fall in line with what we know about JJ throughout the years: for all of his cheapness and irascibility, Jonah has pretty much always been a man of integrity who values hard work, tries to fight against evil and stands by what he believes in. And even if he is a blow hard who works them to the bone, Jameson usually goes out of his way to (low key) help or defend his employees.
Example: Remember in Sam Raimi's Spider-Man when the Green Goblin bursts in, demanding to know "Who is the photographer who takes the pictures of Spider-Man!" and Peter is literally right there? You'd think the smart thing to do would be just to give this inhuman freak the information he wants. But still...without pause, ....Jameson tries to throw the baddy off the kid's trail, even willingly endangering himself in the process.
Yeah, that's right: when facing a super-villian who could snap him like a twig, Jonah didn't cower, thought nothing of himself and protected an innocent. And this isn't a "rare" occurrence in comics or other media; that's just kind of the "often overlooked part" of Jameson's character....
Keep in mind, at this point, Peter had only been working for the guy for a few weeks, at best. But Jameson's sense of justice and his own moral code just wouldn't let him give up a kid---- especially not someone under his ambit---- to an obviously dangerous, violent criminal. He straight up denied knowing the "mystery photographer" 3 times; not only was that loyal, but it was brave as hell considering that the Goblin literally had him by the neck!
Next, it's been implied that JJ "sees a lot of himself" in Peter; Jameson started at the Bugle years ago as a "Cub reporter", and has (in his own way, usually involving a lot of shouting) tried to foster Peter to do his best at it; bring in the best shots, report facts (save where "Spider Menace" is concerned) and use the "Power of the Press" to actually do some good. Even in the early comic days, he'd encourage Peter in subtle ways, as it mutually benefited them both.
I believe it was Robbie Robertson who pointed this out to Peter as a teen, but by the time he'd gotten to college, Peter had developed a genuine respect for Jameson. It's one of the reasons why, even when he was working as a teacher and didn't necessarily need the money, he still would take pictures for the Bugle.
Which brings up the other point: Even though he technically "freelances" as a photographer, Peter has always been "Bugle Staff" at heart. He likely views the Daily Bugle not only as an honest news house, but also as his "home", the place that kept him going financially for years and gave him his start in the business. This is likely why he'd tend to give them "first shot" at buying his work over other media houses; after working with them for between 10-15 years, that kind of allegiance to the company is to be expected.
But it's more than just "professional loyalty" to the paper I'm speaking of here; Peter's loyalty to Jonah himself runs very deep, even given their personal differences. Peter will be the first person to admit that JJ is a cheapskate, a demanding boss, and an occasional pain the ass...but he's also stood up for Jonah against the likes of Wilson Fisk and others who have come against him. Peter, Robby Robertson and sometimes Betty Brant are usually the people to let others know that there's always been more to J. Jonah Jameson than a cigar-chomping skinflint.
In terms of their personal relationship, as well as business, again... Jameson has backed Peter's endeavors towards growth, almost in a parental way.
In various instances, he has (both directly and secretly), aided Peter when he's needed it; he has sought out medical treatment and even helped to pay for May Parker's expenses; he chipped in for elements of the Parker's wedding; at one point (I searched for a reference, but couldn't find it) when Peter Parker was on trial, it was Jonah who paid his attorney fees....there's even a version where JJ was willing to fund a young Parker's college education.
All of these examples again show that, deep down, Jonah is a good man, and that he has developed both a fondness and respect for Peter Parker over the years. That kind of decency and care would naturally spark a certain amount of respect an fealty in a guy like Peter, who initially had so few people in his corner.
As Pete became an adult---- and Jonah's "crusade" against Spider-Man became a "norm" and he grew ever grouchier---- Peter would get even more evidence of his boss/mentor-friends underlying good nature. As such, even when JJ was at his meanest and most crochety, Peter rarely took it personally because, by then, he knew Jonah was someone he could trust, and even count on. Yes that relationship grew from avuncular to more of a man-to-man respect due to age, but the uncle/nephew dynamic never really totally changed... save for Peter becoming a bit more of a "good humored troll" to Jonah.
Not only did Pete have the knowledge that he was essentially making money by taking selfies, but as an grown up, he was less intimidated by JJs bluster. So he would purposefully poke fun at the old guy's grouchy, almost tsundere nature. That kind of teasing usually only happens between folks who are close or have a good rapport....
This personal regard and closeness doesn't just extend to JJ, though; on some level, Peter probably has similar feelings for all the Bugle senior staff.
Considering everything they have been through together, this again, makes sense; Peter dated Betty Brant for years, and even when she married Ned Leeds---a Bugle reported and also friend---- he stood up for them at their wedding. He's always been close to Joe Robertson, seeing him as one of his "best friends" and wise counsel, and he's on a first name basis with a lot of long time workers at the paper. So even if Jonah himself left the Bugle, but these people were still there, he would likely still consider the paper a sort of second home.
There's a lot of reasons all of these people mentioned have been included on the lists of "Peter Parkers closest friends and loved ones" for years...
Some of the non-hero people who have had the biggest impact on Peter Parker's life, including (from left to right): Capt George Stacy, Harry Osborn, Gwen Stacy, Mrs. Anna Watson, Mary-Jane Watson, Aunt May Parker, Ned Leads, Betty Brant-Leads, Joseph "Robbie" Robertson, and of course, J. Jonah Jameson
In very real ways, the Bugle staff is very much like an extended family to Peter. Remember that, after Uncle Ben's death, Aunt May was the only family he had; as a teenager, he had few friends; as a crime fighter, he wasn't part of any group like the X-Men or Avengers until his college years (either the Spider-Friends or a Reserve Avenger, depending on continuity). By his late teen years, though, the Daily Bugle was full of people whom he had developed close ties with, yet weren't "directly" involved in his life's craziness. To Peter, that newspaper is more than a "job"; it's a space he shares with a group of people whom he could count on, and who respect him.
They even formed some of his core guests at his wedding to Mary Jane, along with now best-bud Harry Osborn and "Frienemy" Flash Thompson...
In this context, Peter's loyalty--- both to Jonah, and to the Bugle--- is more understandable.
Speaking of weddings, and of how close Peter is to Jonah personally ...they were even legally related at one point! JJ's dad, J. Jonah Jameson Sr, was actually married to Peter's Aunt May for a while!
Both men participated in the wedding, with JJ even officiating! Peter then, like the quick-witted troll he is, joked about them being "related now" so, "can I borrow 50 bucks?" , which naturally made Jonah almost blow a fuse right there in the ceremony...
Just look at the shit-eating grin on Peter's face; long time readers cannot deny that on some level, Peter Parker has a lot of love for this man. Even comics made after this point show them getting along very well...
Concurrently, there are even What if? comics that show them working together as father and son. In this alternate universe tale, both Aunt May and Jonah's son, John, die as a result of a space shuttle crash; this leaves JJ without a child and Peter without a parental figure. So Jonah ends up adopting Peter, and... while Pete isn't initially too happy about it... by the end of the story, he and Jonah actually become a crime-fighting duo, with JJ encouraging the "Spider Man" to show the human face instead of hiding behind a full mask (one of the problems 616 Jonah has with secret heroes in general, and Spidey in particular).
Now to be completely fair, this doesn't mean these men haven't had serious issues with each other or that that loyalty hasn't been tested. I'm going to quote a quick article written on it where I got the picture:
Before the "One More Day" storyline blew Spider-Man continuity to smithereens, Spider-Man's secret identity was public knowledge as the result of his unmasking in Civil War. This came as a massive shock to Peter's longtime employer, who promptly collapsed from the reveal. Not only that, but he fired his longtime Editor in Chief Robbie Robertson for his defense of Peter's activities as Spider-Man.
Upon learning this info, Peter confronted Jonah in an abandoned building, and their decades-in-the-making confrontation unfolded. By the end of it, Jonah dropped the lawsuit that he filed against Peter and he rehired Robbie back. Oh, and he beat the hell out of Spider-Man as Peter let him vent all his frustration out on him. Naturally, Peter left the scene unharmed, but we're sure Jonah would want the scoreboard to read Jameson 1, Spider-Man 0.
The fact that Pete would face Jonah and let him wail on him just to work things out between them says a lot about their relationship; it meant enough to him that he would face JJ head on, and find a way to work out their differences.
Now to my understanding, JJ is no longer with the Bugle, so if Peter is still peddling his pictures there, it would likely be only because of his standing reputation and relationship with the paper itself. However, despite old cotton top not being publisher anymore, his regard for Jonah remains still ever present; this was shown in Spectacular Spider-Man #6 by Chip Zdarsky, when Peter unmasked himself to JJ ....
A large reason why Jameson hates Spider-Man is that he can't put a face to what is, technically a law-breaker. However, at present time in the comics, Jameson is actually Spider-Man's largest public supporter. Why is that?... Spider-Man finally agreed to a sit-down interview with Jameson. They had the expected back and forth throughout the issue, but it ended shockingly with Peter revealing his secret identity to his former boss. With this new knowledge, Jonah finally put the pieces together and became Spider-Man's biggest public ally.
I haven't been keeping up as much, but my guess is that even if Jonah, Robby and Betty are no longer with the Bugle, he'd still give the paper "First crack" at some pictures or a story (after Jameson's podcast, of course) due to his long standing history with the paper. Peter has always proven to be both trustworthy and a stead fast friend in most iterations, so simple loyalty to the paper that has done so much for him over two decades----if not to the people in particular who mean so much to him-----would likely be a way more major motivating factor than mere money.
Then again, the guy does own Parker Industries, right? So unless something has changed, I doubt money is still a real issue to Peter anymore, anyway.