Assume they should be. Other things being equal, it's safer to crash while buckled up and many people wouldn't wear a seatbelt if they didn't have to, so an enforced seatbelt law should reduce the number of road deaths right? Not necessarily, people may choose to drive faster or more recklessly instead. So other things are not equal; people change their behavior. When America made seatbelts compulsory motorists had more accidents, killed more pedestrians but got killed at about the same rate.
Does this affect your opinion about seat belt laws? One response is that the law is fine, but people should drive as carefully as they did before (or more carefully!); then there would be fewer deaths. That's true, but if scolding motorists into new attitudes is a viable strategy, why do you need the seatbelt law in the first place? Surely you can just scold drivers into buckling up? When making new laws you can't simply will people to change their behavior in ways you find desirable.
Should seat belts still be compulsory?