And practically every subway system already has one-way turnstiles that can "detain" you in the sense that you can't pass until it opens, and either can't return through it without re-inserting a fare card or can't return through it period.
In fact, there's a fuck ton of controlled-access places with systems which effectively do this. What are you already doing for the "detainees" who already are created by those?
(not that I like the TSA, I just also don't see why you're treating this as new or unusual)
Subways that I have seen that require fare cards on exit do not actually prevent you from leaving; you can jump the turnstile or go through a gate on the side that possibly alarms. This is not the greatest thing, but at least while you are inside you can move freely inside a large subway system; being encased in plexiglass until a light turns green is a whole different degree of trapped.
Edit: And I guess this is an important part of the point: degree matters. The old boiling-a-frog story is just about slowly increasing the degree of something.
Have you ever ridden an elevator from the lobby of a large office building? If the security clerk at the desk thinks you're a threat, he'll let you get in an elevator and then press a button to stop you between floors.
When help arrives, they'll override the controls and force the elevator the ground where the only exit is into the hands of an armed response force.
The last time I was in Atlanta, they had a similar thing, but more subtle. If I remember correctly, they have a bank of escalators, but all of the escalators go in the same direction. As I found out when I got to the top, and had to go through security again to get back down...
I can very distinctly remember going through these in FRA.
Because it is default allow the feeling is quite different to the system shown in the video where it feels like default deny and it asks you to wait.
When you exit the secure area of the airport, you usually pass through a corridor with tons of CCTV cameras and a TSA employee. Those cameras and that employee are to keep you from turning around and re-entering the terminal once you've begun the process of exiting.
When you are passing through such an area, you are free to move unless you are detained, which in theory would not happen without good reason. The idea is that you are free to exit and just cannot go back in.
When you enter one of these chambers, you are detained by default until you are released. You are not free to go in any direction. It is very different.
edit: the operative word in your sentence is "_like_" and "_never_"
My point is, you have encountered systems "like" this, if you have ever exited an airport secure area. And it's the secure area, not the airport building itself.
also; boiling frog analogy is quite flawed
As do many fast food restaurants and other buildings, I'm fairly sure it's meant to function as an air-lock, mitigating the equalization of outdoor and indoor temperatures and preventing drafts into the building. Additionally, you are not being detained in such a situation as it is up to your own agency to either manually open the door or activate the IR sensor. In the airport situation it does appear people are being detained because they don't seem to have the option of opening the exit door themselves.
In fact, there's a fuck ton of controlled-access places with systems which effectively do this.
Sure, if someone or group is occupying a building and they don't want people wandering around willy-nilly they can control access and 'buzz people in' or whatever. The airport situation is entirely different as it is essentially controlling access to the entire world, restricting ones freedom of movement.
So all in all, put a little thought into what your saying before a reflexive "nothing to see here."
Every place I've ever seen one of those doors, when you can't return, you can always go forward, when there is some check, and you may not be able to go forward, you can always return.
I guess, the police would be able to legally create a silo that may detain you, but nobody else can (at least on most democratic countries, I'm not sure about the US specifically).
You are correct that many subway systems have something in place to verify that you were a fair-paying customer. It just doesn't strike me as equivalent.
In the subway case, it is there to prevent bad actors from cheating the system. It's lame the same way that people checking your receipts at Sam's Club is lame. In the airport case, you're assumed to be a criminal even before you've received any benefit.