To get it through security somewhere with metal detectors. That's probably the only reason to specifically fear a 3D-printed gun in a nation full of proper guns.

Of course, 3D printed plastic ammo isn't likely to be very effective.

(Maybe they're worried that before long, 3D printing with metal will almost as easy and affordable as plastic 3D printing is now, and people will be printing off entire arsenals of very effective firearms?)

So what are you going to do behind the metal detectors with your plastic gun and no bullets? If you want to do huge amounts of harm (and kill yourself in the process) in the US it’s pretty clear you can do that without the need of a slow plastic gun that may just explode.