I disagree that there needs to be legislation for this in the same way I don't think we need to legislate seat belts, but this is lower on my priority list of 'shits given'. I'm picking my battles, and this is not one of them.
While I agree with this, the problem comes in when other people's negligence can cause someone harm. I may be way off base here, but I would think that if a dog jumps out the back of a truck and causes an accident, there would need to be a law stating it was required in order for the driver of that truck to be found negligent. Also, you can thank the insurance companies for the seatbelt laws. It has nothing to do with the government attempting to save lives. It's about reducing risks so insurance companies can make more money.