The same reason as the Americans did on the Garand, M14 and M16.I have no idea why the Chinese never used a spring to return the firing pin into battery instead of a cartridge.
Read about David Olofson, who was found guilty and sentenced to 30 months in prison for possessing a malfunctioning AR15.
No, he was not. Did you even read the link you provided? What you are claiming isn't even a law, LOL.
He was found guilty, according to your wiki source, of transferring a machinegun. It wasn't simply that it was a malfunctioning gun, but one that he knew was malfunctioning, one that he knew had a problem in the 3-round burst position.