Do you really think that, even if widely available, this would be used more often for crime than for harmless purposes like simply experimenting with 3D printers?

You can experiment with 3D printers just as well by printing toys for kids or plastic roses, simple machinery and other gizmos. Once you decide to print an unlicensed, unregistered gun and get ammo you'd better be prepared to use it for something because in a large number of places you've just crossed into illegal territory which would seem to be a lot of trouble to go to just to 'experiment with 3D printers'.

I don't think that you'll get much mileage out of that line for your defense when you're caught with a stash of illegal weapons.

No, your honor, those 25 guns are misprints, really...

I'm not much of a gun enthusiast. I don't own any. But I certainly see the appeal of 3D printing a gun just to get it to work, because it's cool. The same goes for other things I think are cool, like jet engines, stirling engines, airplanes and multirotors, trebuchets, 3D models of fractals, complex gear systems, etc. I'd be interested in printing all of these things just as a hobby, and a 3D printed gun is no different. There's no reason why a gun is unique, such that I or the average 3D printer hobbyist would only be interested in printing one to commit a crime.