What it is an extra charge so even if you fail utterly at being a terrorist and the fbi fucks up, they can get you for something, lying on an official form
"Well.. what do we have here? Someone that tried to murder people for political reasons but the thing that really got us was when you lied. This is the USA and the one thing that people never get away with here is lying!"
Realistically, probably the same thing as what happens at self-checkout lines with idiotic anti-theft checks:
A beacon above the kiosk turns on, an attendant walks over and, without a word, flashes a badge at the machine to override the system lockout as they roll their eyes: this is only the hundredth time today someone accidentally pressed the wrong button, because what kind of terrorist would voluntarily press "yes".
Except that they take shit very seriously at airports. Try jokingly saying that you've got a bomb while they're checking your luggage, they'll take you away for a few hours.
I asked my girlfriend who works in aviation security auditing I asked her what would happen and she said "you wouldn't be allowed in" as it's part of a visa questionnaire.
Meanwhile ... loading crews are loading an entire cargo box under the aircraft filled with god knows what because those things are packed under weird agreements between airlines, airports and cargo companies ... those cargo boxes could be loaded with just about anything, bombs, guns, pigs, cadavers, severed heads, gold coins, computers, spaghetti, pine nuts or zombies tied in chains.
I wonder if it's a last-ditch psychological effort to dissuade anyone from doing something drastic.... If this has been studied and it's a point of introspection that may let one's conscience reach them.