The scanners don't work. Many explosives can slip through. Do you realize how many fascist-excusers don't know that?
Even if they were to work though, there is still a huge civil liberties issue, a constitutional issue, with denying travel to law-abiding American citizens who do not want to be subjected to such dehumanizing tactics.
If someone has been up to something truly suspicious, let the authorities go to a regular, open-court judge (no secret courts, which are the stuff of the Star Chamber) and obtain a warrant or order. Also, make the person who is the subject of that aware, and allow that person a full, due-process opportunity to clear his or her name.
Only under the rarest of circumstances should anyone not be informed of the specifics of why he or she must under-go extra procedures or be banned from flying, etc. Even then, there should be a clear time-limit for the government to either bring charges or back off. Otherwise, we are on the path to a police state.