Last night I was watching the TNG episode "Relics", where Riker, La Forge and Worf beam aboard a disabled 75-year-old ship with minimal life support. Upon arrival, Riker takes a tricorder reading and reports that the air is "stale" but breathable. Outside of the obvious reasons of production costs and actor comfort, is there any reason why away teams would be allowed to beam into a relatively unknown environment without being required by regulations to wear an environmental suit (or at least bring along some spare oxygen)?
I'm not sure of how "unknown" the atmospheric conditions aboard the ship were. They always scan for an atmosphere first and Starfleet officers trust their technology almost implicitly. If they beam over then they're probably already confident that they are not in danger. The tricorder checks are just to make sure.
There reaches a point when you have to assume that they have the same common sense that you do, even if it's not shown on-screen.
Besides, the transporter keeps the story moving faster than a shuttle.
I think you may have misunderstood the scene.
As you can see from the show script, at the point that they beam over Data has already taken a scan of the ship's atmosphere and determined that the air is breathable.
Data : There are no life signs. However, there are several small power emanations... and life support is still functioning at minimal levels.
Once they beam over, Geordi gets out his tricorder. This isn't to check that the atmosphere is breathable (which they already know from Data's scan) but to perform a closer inspection of the ship's life-support systems:
RIKER : (reacts to smell) The air's pretty stale.
GEORDI : (off tricorder) Life support is barely operating.
RIKER : (to Worf) See if you can increase the oxygen level.
As to why they don't routinely wear respirators on away missions, the answer is simple; There's no need if their scans show a breathable atmosphere.
Clearly, in a real life scenario like this, they would have to take some significant precautions before boarding such an environment.
However, in universe, they do a check and verify that the atmosphere is suitable - which it seems to be most of the time ( clearly the life support systems in ships are some of the most relaible systems ). They also know that if there was a critical problem, they could beam back straight away, which would help. And they do the checks when they are present to verify the exact make up of the air, and how much is available.
Given that level of technology, it is probably OK for them to go without any extra support.
I guess it depends on how 'canon' you consider the animated series. In some episodes, The Slaver Weapon being the one I remember most distinctly, crew members are seen wearing kind of a force shield that acts as an environmental suit. Other races are shown wearing a standard environmental suit.
Also, in Star Trek: First Contact Picard, Worf and Hawk go on a EVA to the deflector disk in environment suits. Not exactly an away team though.