The TSA is probably not using these reasons, but there are security reasons to screen everybody. Any time you make an easy path through security, you invite your adversary to use it. You talk about screening grandmothers as if this was irrational; do you think there are no old ladies out there who harbor a deep hatred of the United States and are willing to carry a bomb onto an airplane? There certainly aren't a lot of them, but they don't need a lot, they just need one.