So the State Department is attempting to suppress the blueprints for 3-D printable firearms ...
You can hold any opinion you like about the propriety or morality of spreading the information about how to create your own weapons via 3-D printers around the planet, but at this point (sorry if I offend your sensibilities) your opinion is meaningless.
The information is out there. It has been downloaded and copied to mirror sites (many far out of reach of the US government) millions of times.
Suppressing knowledge, even potentially dangerous knowledge, has always been a fool's game, which is to say: something the government constantly attempts.
The height of irony, of course, is for the State Department to take this action under the pretext that publishing the blueprints violates arms trades regulations.
The State Department presides over the largest arms trade operation on the planet, and will sell virtually anything to virtually anybody. There are multiple conflicts now where US arms are being provided to both sides.
So instead of believing that the State Department is somehow motivated by the idea of keeping us all safer, my best guess is that its clients (the arms manufacturing industry) demanded that action be taken to keep their profits safe.