From the link above:

Background provided by TSA:

"The advanced imaging technology scanner at the checkpoint helps TSA identify concealed metallic and nonmetallic items between the skin and clothing using millimeter-wave technology. So if an individual were to try to conceal something in the area of the groin, the machine would detect it. It is not out of the question that the machine could detect something placed inside an individual's underwear."

TSA just doesn't see the problem with a punitive search. This level of search is consistent with being a prisoner.