They got wind it was to be used to make a gun, and terminated his lease, suggesting legal issues. We talked about this some here. I’m not sure the company is wrong, as a matter of law, even if the concern is absurd technically:
Wilson’s plans may have fallen under this review, which could have provoked Stratasys to pull the lease. There’s also another law, the Undetectable Firearms Act, which could mean a fully plastic pistol would be illegal regardless of how it was made. Guslick’s partly plastic AR-15 seemed to have circumvented this law by only building one component of a mostly metal rifle with 3-D printed parts.
This was passed in the 80s during the Glock scare, where people bought the myth that the Glock was some kind of undetectable plastic gun. A plastic receiver is certainly legal, but a completely plastic gun (an impossibility, with current materials, if you ask me) would need to have enough metal in it to set off a metal detector calibrated to the “security exemplar” in order to be legal, no matter who manufactured it. However, I think a completely plastic gun would be a grenade rather than a firearm.
The security exemplar is legally 3.7oz of 7-14 stainless steel shaped like a gun. In addition, the “major part” of the firearm would need to appear on an x-ray to be readily identifiable as that major part of  a gun. The law is very ambiguous on what “major part” means. My understanding is that, for a gun like the Palm Pistol, the problem was dealt with by making the frame such that it would spell out “GUN” to any airport screener, despite the shape of the device being non-conventional.
You will note that the Undetectable Firearms Act is an exercise in poor lawmaking, and is represented in the United States Code under Title 18, USC 922(p):
(p)(1) It shall be unlawful for any person to manufacture, import, sell, ship, deliver, possess, transfer, or receive any firearm –
(A) that, after removal of grips, stocks, and magazines, is not as detectable as the Security Exemplar, by walk-through metal detectors calibrated and operated to detect the Security Exemplar; or
(B) any major component of which, when subjected to inspection by the types of x-ray machines commonly used at airports, does not generate an image that accurately depicts the shape of the
component. Barium sulfate or other compounds may be used in the fabrication of the component.
(2) For purposes of this subsection –
(A) the term “firearm” does not include the frame or receiver of any such weapon;
(B) the term “major component” means, with respect to a firearm, the barrel, the slide or cylinder, or the frame or receiver of the firearm; and
(C) the term “Security Exemplar” means an object, to be fabricated at the direction of the Attorney General, that is –
(i) constructed of, during the 12-month period beginning on the date of the enactment of this subsection, 3.7 ounces of material type 17-4 PH stainless steel in a shape resembling a handgun; and […]
It continues beyond here, but this is the meat of it. You will note that it specifically forbids considering only the receiver or frame of a firearm from being considered the firearm. But it does allow the frame to be considered as a major component when considering the whole. So the way I understand it is, you can’t consider the frame itself to be considered a “major component” unless it’s coupled with the rest of its part, in which case the frame may be considered. Clear as mud?
Remember, our opponents tell us firearms are less regulated than teddy bears. The reality? I think this basically says the frame or receiver itself doesn’t matter, so polymer receivers are OK. But considered as a whole, minus grips stocks and magazines, it sets off a magnetometer calibrated to the exemplar. Further, when considered as “major components”, that the the components don’t obscure themselves to an x-ray screener. In other words, that they look a gun or the major components of a gun they are meant to represent.
My theory is that this is part of the phenomenon of the academic community approach to individual freedom and due process jumping the campus border into the real world, perhaps first seen in the Duke Lacrosse case. There’s no rational explanation for why they’d take such an interest in stopping someone from using their product.