I used my seat belt before there were laws stating you had to do so. Why in hell would you need such a law in the first place?
If accidents caused by stupid resulted in their own injury and death without affecting the rest of us, I'd be in agreement with you - that's Darwin at his good work!
Unfortunately, stupid often kills or injures other people and when stupid doesn't put on his or her seatbelt and crashes non-fatally, we all get to pay to patch up their bloody carcasses.
Seatbelts, airbags, side-intrusion bars.. all there to mitigate against the actions of stupid (and self), because we worked out long ago it's less expensive for society if stupid doesn't horrifically mangle themselves and others when they inevitably do something .. stupid.