It is a typical NRA ploy to insist that a armed group of people will deter or fend off a attacker. This is just patently stupid.
As far as why you should be able to buy a AR-15 I am with you on that.
You should not be able to buy a Claymore antipersonnel mine or a AR-15 for the same reasons imo.
"You were not there for the beginning. You will not be there for the end. Your knowledge of what is going on can only be superficial and relative" William S. Burroughs