Well, let me ask this: What if an inner city police department said "We are tired of all the gang violence in the area of 1st Street and Main. SO......from now on, every single person who drives or walks in a 4 mile radius of 1st and Main will be stopped and searched for weapons."
That would be a 4th amendment violation. You can't stop every single person and search them based on the notion that past crimes have occurred here, and we know if unchecked future ones likely will also, thus we're stopping everyone.
Thats basically what the gov't is doing with the TSA. And it is indeed a 4th amendment violation.
Disagree? Read 4th amendment case law.
A person is protected against unlawful search or seizure without a warrant, or without probable cause that the individual has committed or is about to commit a crime.
So, Joe Blow plane passenger enters a terminal. No warrant to search him exists. And the TSA agent has absolutely no probable cause that Joe has committed or is about to commit a crime. Thus, the search is a violation of the 4th Amendment.
But Americans aren't fighting very hard for their rights now. So....there you have it.