That's completely wrong. There is absolutely no criminal intent if they hold a gun which they can not repair to a safe condition. There is no crime. In the interaction, they are legally held to a much higher standard of knowledge and responsibility. If a car dealer was servicing your 1963 BMW and noticed the rotors were worn past the safe limit but didn't fix it because they could not get parts for such an old model.... they would be liable for any crash related to brake failure. I have actually seen similar cases: they will type it onto the service order and require customer signature understanding the risk before they will release the car. With a gun, catastrophic failure is so dangerous that the maker will simply not assume the risk of returning a defective gun to service if it is in their hands. And you know why:

"Ladies and gentlemen of the jury, this death weapon from hell had been serviced by the maker only nine years prior to the incident and the manufacturer did not disclose the risk to my poor client..."