The US government has tried to apply its arms export control rules to 3D model files that describe firearms, and declare that publishing those files is the same thing as exporting guns, and is therefore prohibited. Whatever you think about 3D printed guns, love 'em or loathe 'em, that's a terrible way to deal with them.
Applying the International Traffic in Arms Regulations (ITAR) rules to online publication of mathematical descriptions of 3D objects is effectively demanding that Americans get a license to publish from the government. Not just any license either: under ITAR, the government has no objective standards that it has to follow, nor any timeline for responding to petitions, nor any judicial oversight for its decisions.
The Electronic Frontier Foundation filed a brief in Defense Distributed v. Department of State, tying the case to its seminal, twenty-year-old victory in Bernstein v US DoJ, which legalized the publication (and therefore the use) of effective cryptography, striking down the NSA's restrictions on its dissemination.
Whatever a legitimate export control regime may look like, it does not involve standardless, unreviewable censorship of all online publications describing entire ranges of technology that have civilian uses, and about which the public needs to be informed in order to evaluate and challenge our government's policy choices. The First Amendment does not prevent proportionate measures to prevent weapons from reaching those who would misuse them, but it does mean that the government cannot choose the quick-and-easy path of broadly criminalizing online speech and figuring out what speech it wants to allow when publishers go to the trouble of asking permission.
In 3-D Printing Case, "Code Is Speech" Faces New Challenges