Does a yakiniku restaurant not need a license just because you cook the meat yourself?
I just find it an interesting idea with no clear line. Who is making the gun, the person who set everything up? The machine? The person who pressed the start button?
Lawyers and politicians will give a spectrum of answers to that.
In reality, which is what should matter, if 99% of the effort went into pre-production so that the final step of production could be made correctly, the person doing the final step is the one who made it.
If I give you a software program that can crack a site's user password list, and you run it on a selected site, am I the one in trouble? Only if I knew you'd use it illegally am I in anyway accountable. Since there are legitimate uses for these types of utilities, you can't say the only reason for the password cracker was for illegal purposes.
Same for the lowers. As long as I do everything legally to set you up to make s usable lower, and you do the last step that turns an 80% into a 100% workable lower, I did nothing wrong. If you then use that lower to break the law (Ghost gun, robbery, mass shooting, ...), and I had no inclination that was your intention, I'm no more at fault than a store selling rat poison that someone uses to kill their spouse.
At some point, common sense gun control needs to actually start using common sense. The plans to 3-D print a gun are available. If someone downloads and uses the product illegally, is that the CAD designer's responsibility? What about the 3-D printer manufacturer? Or the raw materials used on the printer to form the gun? How far back can we "catch" someone about to something wrong?
Maybe we should apply this same logic to public welfare. Billions of dollars are stolen through fraud using the various government assistance programs: Medicaid, Medicare, Food Stamps, Stimulus Loans and handouts, etc. Should we put the people in charge of these programs in jail for the fraudulent losses these programs experience? Are they liable for the actions of others, just because these people made the funds available and they were misappropriated?
Same thing in my opinion. Either we are all guilty of what others do when we are enabling them, or we are not. Unless I know the ONLY REASON people are using my resources is to break the law, I can't be held criminally responsible.
Negligence is another aspect of this, but that gets into whether complying with the letter of the law is sufficient to prevent bad acts by someone using your product or services.