At airports, one does not have a choice, in order to commute, does one?
If one refuses to be searched in an airport.....one does not get on the plane.
Airlines are private, subways are public. Like sidewalks. if they can leagally search you on the subway without probable cause, they can legally search you without probably cause on the sidewalk.
The 4th Ammendment prohibis that.
Because everyone should have something to hide.