Ronnie's one comment speaks volumes about why the TSA gets away with this:

They have never been forced to state what is a threat and what isn't. I think the cop community calls this the "elephant behind the bookcase" situation. When real cops conduct real warranted searches, they have to state what they are looking for. If they say they are searching for elephants, they can't discover any evidence of a crime (the legal definition of "discover") in an area smaller than one would reasonable expect an elephant to be hidden.

The TSA has never been compelled to define the smallest prohibited item. Because of the benefit of their ability to search and define anything as a prohibited item, no law enforcement agency on the planet will stop them from doing the work they can't legally do. So, until compelled by law (not that the TSA would obey the law anyway), the smallest element in the known universe -- an atom of hydrogen -- can be determined to be a prohibited item. Since it's the smallest substance in the universe, it can be hidden anywhere in luggage or on an individual's body. This gives the TSA license to search anywhere, anybody, anytime.

The TSA has also never been compelled to articulate their reason for searching a bag or an individual. I simply can't believe that a clerk looking at x-ray images of carry-ons does not know what a joint, a bundle of cash or bag of drugs looks like. They never have had to justify a search beyond stating, "I think I saw something." There was one court case where the TSA had the stupidity to authorize the clerk herself to testify. (I think it was the case of the guys carrying a lot of credit cards or something.) The clerk admitted under oath that she had no security reason to search a large envelope. As I recall, the judge actually followed the law enough to throw out the evidence. You can bet that the TSA will ensure that no other clerk ever gets even close to a witness stand again, except as a defendant.

Geez -- I want out country back!