Why are people surprised that Hogwarts is non-apparateable?

Question

Most Wizarding dwellings are shielded against Apparation.

“Professor, why couldn’t we just Apparate directly into your old colleague’s house?”
“Because it would be quite as rude as kicking down the front door,” said Dumbledore. “Courtesy dictates that we offer fellow wizards the opportunity of denying us entry. In any case, most Wizarding dwellings are magically protected from unwanted Apparators.
_{(Harry Potter and the Half-Blood Prince - Chapter 4)}

Yet everyone in the Wizarding world apparently assumes that Hogwarts would be different. Why is this?

All around them, people were asking one another the same question: “How did he get in?”
“Maybe he knows how to Apparate,” said a Ravenclaw a few feet away. “Just appear out of thin air, you know.”
_{(Harry Potter and the Prisoner of Azkaban - Chapter 9)}

“Harry!” squealed Hermione. “What are you doing here? How — how did you — ?”
“Wow!” said Ron, looking very impressed, “you’ve learned to Apparate!”
_{(Harry Potter and the Prisoner of Azkaban - Chapter 10)}

“He must have Disapparated, Severus. We should have left somebody in the room with him. When this gets out —”
_{(Harry Potter and the Prisoner of Azkaban - Chapter 22)}

“A Portkey?” Ron suggested. “Or they could Apparate — maybe you’re allowed to do it under seventeen wherever they come from?”
“You can’t Apparate inside the Hogwarts grounds, how often do I have to tell you?” said Hermione impatiently.
_{(Harry Potter and the Goblet of Fire - Chapter 15)}

“I don’t think so,” said Harry, shaking his head. “He seemed really weak — I don’t reckon he was up to Disapparating or anything.”
“You can’t Disapparate on the Hogwarts grounds, haven’t I told you enough times?” said Hermione.
_{(Harry Potter and the Goblet of Fire - Chapter 29)}

“So he did Disapparate?” said Ron.
“You can’t Disapparate on the grounds, Ron!” said Hermione.
_{(Harry Potter and the Goblet of Fire - Chapter 29)}

“One day,” said Hermione, sounding thoroughly exasperated, “you’ll read Hogwarts, A History, and perhaps that will remind you that you can’t Apparate or Disapparate inside Hogwarts. Even Voldemort couldn’t just make you fly out of your dormitory, Harry.”
_{(Harry Potter and the Order of the Phoenix - Chapter 23)}

Why is everyone surprised that Hogwarts is non-apparateable?

Answer 1

Let's review what Dumbledore said about Apparating:

In any case, most Wizarding dwellings are magically protected from unwanted Apparators.

I see a loophole: 'unwanted'. The Weasleys seem to have no problem Apparating within their own house, and I assume that the Weasleys have most standard magical protections. Furthermore, Mrs. Weasley seemed upset enough about Fred and George to have been willing to block Apparitions within the home if that was an easy option.

Let's consider now the people who were thought to have Apparated within Hogwards: Sirius Black, Voldemort, and Harry Potter. Hmmm, two of the most notorious Dark wizards of the day and the Boy Who Lived. (Sirius Black was considered a notorious Dark wizard by the people who thought he (Dis)Apparated.) If anybody is going to find a way to evade an Apparition block, it's one of these three.

Now, with an awareness of this loophole and the ubiquitous nature of Apparation shields, you can see why only a detail-lover like Hermione would pick up on the fact that Hogwards doesn't just have an Apparition shield, it has a unique and extra-powerful Apparition shield which has no loophole to exploit or trick. Thus, people keep mixing up the Hogwarts shield and the type of Apparition shield that they've been dealing with all their lives, and this mix-up is the source of people's recurring confusion about Hogwarts and Apparition.

To a large degree this is hypothetical, since we really don't get much information about Apparition as a general-purpose thing.

Answer 2

I don't think everyone is surprised. The only people who mention Apparating into, or Disapparating out of, Hogwarts are students (generally Harry and Ron, repeatedly). Why would students be surprised/not know that you can't Apparate into Hogwarts?

They don't read Hogwarts: A History

The only place that particular piece of information is ever mentioned as being recorded is in the book Hogwarts: A History. Up until Harry's second year Hermione seems to be the only student who has read the book and remembered parts of it. Even in Harry's second year people likely would have only read the parts concerning the Chamber of Secrets (since that was why they all checked it out of the library).

They're unfamiliar with Apparating

For Muggle-borns this is obvious: they're unfamiliar with Apparating because they're unfamiliar with almost everything relating to the Wizarding world. For those born to at least one magical parent, they may not actually know anybody who does Apparate. We know that it's dangerous, that you have to pass a test in order to do it, and that a lot of witches and wizards don't bother with Apparition, preferring slower but less hazardous forms of transport.

As a result, many students are likely only familiar with Apparating on a very, very basic level. They know what it allows you to do but have no idea how it works.

Different expectations between a public place (a school) and a private dwelling

At least parts of the Ministry of Magic can be Apparated to. The Ministry of Magic is, essentially, a public place, since it accepts visitors and not just government employees.

If you can Apparate to the Ministry, why wouldn't you be able to Apparate to Hogwarts?

They've just never thought about it

If your home has been magically protected against Apparating, unless you're explicitly told that's the case you're probably never going to think that it is. Even if you notice that nobody ever Apparates into your house, you'd probably just think that wizards Apparate to a spot outside your house because that's good manners, not because they're physically unable to do anything else. I think it's unlikely anybody would ever notice that nobody Apparates into their house, though.