It’s entirely fair to claim or insist that 3D printing of guns is going to lead to more than the occasional problems of untraceability. That is indeed the claim being made by various gun control groups. Depending upon your Second Amendment views, you can view the ability to download plans and instructions as a good or a bad thing. But it was always obvious to me that courts would rule only one way – it’s legal, entirely so.
Americans are pretty much allowed to say what they like without prior restraint. Sure, there are laws about what you cannot say (libel, incitement to immediate violence, and the like), but they’re always applied afterwards. Anyone wanting to attribute blame or responsibility to those who post gun designs online will be equally able to do so. So, people can post such designs, and people can download them. What happens next, well, free speech is part of that tree of liberty that must occasionally be watered with blood.