It is retarded.
You can never regulate it - as you point out, it will not in any way stop criminals.
There are ALREADY laws against owning undetectable guns. We do not need stupid regulations adding nothing.
The republicans are in a pickle. If they support 3D gun printing, they hurt gun manufacturers, which is what they really are supporting ($$$$).
The democrats are in a pickle. If they support banning 3D printing, they are making the case that they are indeed the big government control all party.
How this plays out will be, at worst, hilarious to watch, all the while knowing that nothing can ever really happen, just like DRM.