Most currencies have a special pattern that printers are programmed to detect and refuse to print. Since illegal gun part designs can’t be forced to include a marker declaring that they’re gun parts, a 3d printer would have to 1) know what a gun is, 2) know how a gun works, 3) be able to tell whether any particular shape could be used as part of a gun, and 4) be able to tell whether any particular shape could be cut and reassembled into a shape that could be used as part of a gun
They could make it difficult to open up the camera and extract its signing key, but only one person has to do it successfully for the entire system to be unusable.
In theory you could have a central authority that keeps track of cameras that have had their keys used for known-fake images, but then you’re trusting that authority not to invalidate someone’s keys for doing something they disagree with, and it still wouldn’t prevent someone from buying a camera, extracting its key themselves, and making fraudulent images with a fresh, trusted key.