It's written into the constitution so the people don't get overrun by a tyrannical government. Currently they are not completely overrun, but hey they are used to bail out banks, fund wars to control resources, etc...
Have you read the US constitution? Particularly the second amendment...
I'm no fan of guns, but if you're going to ban it for the public is should be banned for the army and governments.
The problem is not guns, it's the people. The people in Switzerland have guns and their crime rates are as low as they come.
