My guess is that the passengers being 'detained' are simply told by the screeners that they cannot leave. Not knowing the limits of authority they probably believe the screeners are telling the truth (like they always do). In any case, should the passengers do as they are entitled to and exit the sterile area I am quite sure the screeners would call the LEOs to locate the 'perp', probably fabricating some kind of threatening behavior. And 99% of the LEOs will of course believe them.