Because criminals will get their hands on those weapons no matter what you do to get rid of them. That is why they're criminals, after all, they do things that are illegal in the first place. If you disarm the populace, the only person you're really disarming is the law abiding citizen. The idea is that the law abiding citizens should legally have access to the same weapons that their would be aggressors do. How well is an unarmed citizen supposed to be able to defend themselves against a semi-automatic firearm? Sure, if we were all Shaolin Monks we could probably do without weapons, using our hands as forged weapons against any close ranged attack, even with a firearm! But... we're not. We're Joe Schmoe pasty white belly roll Americans/Canadians/English...ans... dude, what the hell is the plural for people from England?