If it's about stopping a random idiot, then the existing laws should be fine.

The problem is that it's a minor inconvenience to determined criminals. At the same time, it's a industry-destroying law for 3d printer companies.

I also really don't think it's about ensuring the profits of weapons companies. Weapons companies are big and already fairly regulated. As a result, it's easy to keep an eye on them and ensure that if there are laws about what's legal and illegal to build and sell in the USA that those companies are following them. I think lawmakers are scared that with 3d printers in the hands of hundreds of thousands of people, it's much more difficult to ensure that the laws are being followed. It's about control, not profits.