I was reading about Schengen transit visas today, and in [an answer](https://travel.stackexchange.com/questions/30569/do-i-need-a-visa-to-transit-in-the-schengen-area) I saw that Schengen requires some people to have an airport transit visa. This visa is apparently for entering the international transit area of airports located in Schengen, i.e. not entering the Schengen area, not crossing any border.

This means that to just get out of the plane that brought you there, you will need a visa. In the answer, it is mentioned that it is the duty of the airline that flew you there to check for this visa, that they risk a fine.

But, in fact, as far as I know, airline crew is not entitled to perform any police/border duty, is it? In many cases they check the passengers' passports to make sure passengers are allowed at their destination because airlines are supposed to carry the passengers back if passengers are not allowed (I vaguely remember reading about this), but I am not sure airlines are allowed to deny any passenger to board for lack of visa.

On top of that, there is no control directly at the exit of a plane in Schengen area, so if a passenger merely transits in the international area, it is not even possible to control this person was there.

So, is this airport transit visa rule actually enforced?