yea it's an interesting one, they also block websites for the anarchist cookbook and most places won't sell the physical book. but some places did and you can totally or at least recently enough could buy it even on amazon.
And they block sharing info for example on how to cook your own meth form OTC pharmaceuticals.
But in those cases, actually following the instructions is directly illegal. Whereas printing/possessing a 3D printed gun isn't even necessarily illegal, by amend no. 2
So, I could see an argument made from those points.
again tho, who needs a 3d printed gun, apart from novelty. in alotta ways it's just protecting people from their own stupidity. It's not an infringement that i'm worried about, but for sure you could argue it's an infringement nonetheless.