Ethics and rights aside, this isn't even possible to comply with. The law applies to additive and subtractive manufacturing hardware, and expects it to automatically detect and refuse any attempts to manufacture a firearm. But that depends on an underlying assumption that a machine can answer the question "will this command (set) result in the construction of a gun?" And that assumption is false.
Even if you somehow designed an algorithm that could read a G-code program and determine whether it produces something shaped like a gun, it still wouldn't be enough - because the CNC machine is just one step of a manufacturing process. The human operator controls what materials and commands go into the machine, so they also have full control over all inputs that the "oversight" program is allowed to see.
Some simple ways to bypass this (hypothetical and perfect implementation):
Print the gun in two pieces, then combine them manually.
Print something that contains a gun, then cut out all the non-gun material.
Use multiple separate machines for different parts of the gun.
Design something that looks nothing like any existing gun, but is still capable of firing a bullet.
Print most of a gun, the build the rest by hand.
Build part of a gun, then print the rest onto the hand-made parts.
Print part of a gun, then run a separate program to print the rest onto the previous result.
A thousand variants of the above.
So even if this was something we wanted to enforce, it's just not possible. For the same reason why Minecraft abandoned their plans for an SMP "penis detector", this law could never be complied with because it's impossible to build a machine that actually meets the requirements.
