It’s too dangerous here. Do something silly wrong, and homeland security might detain you for a few days, denying you sleep and food. (Via Matthew Yglesias, who describes his experiences visiting a sane place).
This is the sort of abuse that only happens under a democracy or a totalitarian regime. A regular authoritarian government is not so stupid, but in a democracy the people who put this policy in place will be lauded for fighting terrorism. That the United States will eventually become a worldwide pariah is none of their concern.