I agree wearing a helmet/seatbelt is common sense. I wear one of 'em every single time I'm on the road. Problem with that line of resoning is that common sense isn't common, nor does it make sense to most people.
What I have a problem with is somebody else telling me what is good for me, or in my best interest. Do you wear an bringht orange traffic vest when you ride? To ride on base, I'm required to because it is safer, prevents me becoming a sploch on the road, saves the AF from having to pay my family and retrain someone else in my stead. Would you like to see that law too? It's just common sense to the powers that govern the military. (Did I just use sense and military in the sentence?)
I believe that if that is a risk someone is comfertable with (kind-of like skydiving) let them take it. If they have been informed so they can make an educated choice, and chose what I feel is the wrong chioce, it is still thier choice. Again, Dawinism.
It might seem heartless, cycnical, or maybe I'm just an @$$. But, we are talking about adults here (the laws all said for those over 21). Let them decide what's best for thier own good, or what risks they're willing to live with, such as wither to ride in a cage, or on a bike. SOme think that's too risky as is. To each thier own.