Are you sure that's it? Or is it really that government sees the armed citizen as a much greater threat to its continued existence than the gang member (a useful tool) is, and punishes the two accordingly?
Were this not the case, armed citizens would have ended gang activity in the US decades ago.
It has more to do with mindset than with citizens being armed. There are plenty of citizens that are armed.
But the police are afraid of both the armed citizen as well as the armed gang- banger. And the judges as well as the lawyers are more in favor of the gang-bangers than the victims. In nearly every case the victim gets the short end in the courts.