The same thing that is stopping them from suicide bombing the super crowded security checkpoint line before ID checks.
Nothing really.
Or going into the baggage claim area with a bag containing an explosive device, then acting like they grabbed the wrong bag and putting it back on the carousel, and then leaving.
We need to put a checkpoint before the checkpoint so that never happens!
Or going into the baggage claim area with a bag containing an explosive device, then acting like they grabbed the wrong bag and putting it back on the carousel, and then leaving.