Once again, the problem here is that all other objects become deadly internal projectiles in the case of an accident as well. If we really cared that much about the danger from projectiles (human and otherwise), then by law, cars should come with multiple tie-downs all over the interior of the vehicle, and it should be illegal to have an object in the car over five pounds not firmly secured by them.

The reason, of course, that that isn't mandated is the same as the answer to all of the other questions in this thread: in the end it's really just about policing people's behavior and choices (and securing an additional revenue stream for cops, as well as a handy additional excuse to pull people over and violate their rights).