Surely this can't be legal?

These are people who don't give a fig about the law. Only what they think they can get away with, and they get away with plenty. Most people are too timid to stand up to uniformed officials and assert their Constitutional rights. People understand that the scum who work for these agencies will cheerfully impose additional inconveniences as a response to any resistance. Most people will do what they're told in order to get away from the shitheels who are able to prevent them from entering the US, regardless of their citizenship status. It's not legal, but it's hard to argue with guns, clubs and locked doors.