Makeshift weapons that can be assembled from one visit to the hardware store are insanely cheaper and less likely to raise eyebrows. Anybody who buys a 3-D printer will be flagged, probably, if they also buy any ammunition and yet don't have a registered firearm. It's just too easy to track and flag. Not to mention insanely expensive. The insane usually don't have several thousands of dollars just lying around.
And anybody who doesn't like the 3-D gun can't do anything about it. It cannot be "uninvented." Nicolas Cage taught us that in "The Rock." Attempts to make the gun not exist will only lead to its further circulation and exacerbate its creators.