I support Florida's refusal to make helmets mandatory for motorcycle operators or passengers over the age of 12, but I never ride without one and every time I see someone riding without a helmet I think "There goes Natural Selection waiting to happen."
Applying this mindset to seatbelts: A case can be argued that a driver not wearing a seatbelt may be more apt to lose control of his or her vehicle in a minor accident that doesn't stop the vehicle, or during a sudden violent avoidance maneuver (though bucket seats and center armrests make this highly unlikely), and therefore a driver's use of a seatbelt has a potential impact on others. Further, I can buy into an owner's responsibility to provide seatbelts for all occupants so that they can choose to wear them if they wish.
But frankly I think laws making seatbelt use mandatory for adults make as much sense as trying to stop psychos from shooting up a school by making certain kinds of firearms illegal: it's an exercise in political expediency that has no positive impact on society.
Convince me that a law justly proteccts others from me, rather than me from myself, and I'll support it. I don't see that with seatbelt or helmet laws, though I think the only sane personal choice for both is to use them.