Originally Posted by RadioGirl

If he's undocumented at the door of the aircraft or jetway, CBP knows which flight he was on and what country it came from. If CBP has the flight manifest and has processed the other passengers, they should be able to identify him by the process of elimination.

Whereas if he turns up at the booth and claims to be on a different flight from a different place, probably using a false name, it becomes harder to sort out who he is.

Doesn't help if he destroys his passport after leaving the plane but before the CBP booth, of course.

I bag TSA (and occasionally CBP) for bone-headed moves, but this one sorta makes sense.