General warrants are unconstitutional, and yet somehow, magically, it's okay to molest everyone at airports without even so much as a warrant or suspicion? Yeah, right.
They aren't searching people for criminal reasons. You aren't under suspicion of a crime, and nothing found will be used to investigate that crime. Theoretically, if they found evidence of smuggling, they are required to pass you through security unmolested (provided you meet all the other requirements). It's for that reason that they are "legal".
I'm talking about people 'consenting' to the search. TSA apologists sometimes make the argument that you implicitly consent to waiving your constitutional rights by trying to get on a plane when you know the TSA is going to try to search you.
You aren't implicitly consenting by getting in a plane I've flown a number of times with no search at all. You are explicitly complying when you walk past the "you will be searched if you pass this sign" sign. That isn't implicitly waiving a constitutional right. That's explicitly doing it.