I agree, the actual cause of victims in an accident, is the accident. But there is a secondary cost as a result of the accident, which is the injury of the occupants of the vehicle. Seat belts increase that injury.
Have to disagree with not being a criminal if you cause an accident; In the US (How is it in Sweden, Eddie? And don't know where you are Unker), you can and often are ticketed by the police for causing the accident; although it is not usually a major crime, things like running a stop sign or losing control are offenses that can be ticketed. There have also been criminal manslaughter charges due to driving negligence when someone is killed.
Yes, true. So this is NOT a victimless crime, and falls outside our discussion. But again it is an action of people that CANNOT harm the government, so the government has no reason to legislate against it.
It seems by constraining you mean taking away personal choice. It does seem that the punishment should be appropriate to the crime.
The penalty for not wearing your seatbelt is only a few dollars fine. I don't think there is a criminal penalty, except perhaps some points on your license. The potential for greater injury if an accident occurs is large however.
The legal penalty for cheating on your spouse is none. Other things could occur, such as venereal disease, divorce, child support, etc etc.