The problem, as far as they are concerned, is enforcement. The law can say that X is illegal and ban selling it, but if you can 3D-print it, and its use is already illegal either way, then the law is moot.

Note that this isn't really limited to guns either. I foresee a similar moral panic wrt 3D printing "killer drones" (i.e. parts that allow one to attach explosive to a drone) and software that allows you to circumvent various restrictions on drone firmware. All it'll take is one high profile attack using a drone...