The potential harm that can be caused by a <2lb quadcopter is utterly microscopic compared to the damage that can be caused by motor vehicles. Yet the regulatory burden on a small quadcopter is an order of magnitude more complex than that of a two ton pickup. You have to juggle the requirements of multiple federal agencies, and state and municipal regulations are a dime a dozen.
In most jurisdictions, the regulatory burden of a small quadcopter is significantly greater than that of an AR-15. The potential harm vs utility argument doesn't even register here.