The problem is that crooks usually don’t maintain their weapons well, so there’s a potential for the would-be victim to end up with a low quality, or unsafe weapon. If this proposal were pursued, what I’d prefer to do is that the weapon gets confiscated and destroyed, and the crook is ordered to make restitution to the victim in an amount that includes the full purchase price of a new weapon of similar make. That way, if he grabs it, instead of getting somebody’s old piece of junk, he gets a brand new gun. We’d also be injecting a little extra money into the economy that way.
Look up the “Broken Window Fallacy."