> So my assumption is immediately that some relatively large lobbying group feels threatened by 3d printing...

I'd say the real groups behind this are the anti-gun ideologues, the "do whatever it takes to stop my panic attacks over Bad Things maybe happening" left-wing control freaks, and the old-fashioned "big state" authoritarian crowd.

And the only reason they're paying attention to 3d printers is that some pro-gun ideologues and provocative makers have been talking up the concept of 3d printing guns.