Your guess is as good as mine, but you’d look like a realllll douche if you were in charge, disregarded a bomb threat, and then someone blew up. I think that’d play a big factor in it.
It may also simply be codified in their policies to treat them all as real threats to avoid any guesswork or considerations. I’m not sure how prevalent these fake calls were when the rules were initially setup, but it’s clearly being abused now and their policies should be rexamined if that’s the case.