The same person or thing that pays the medical bills for any other injury.
Your argument makes no sense. By your reasoning ... we should pass a law requiring everyone to wear full fire protection suits in their homes... because ... who is going to pay the medical bills when they get get massive burns during a home fire?
You can’t protect everyone from everything.
Bikers who I see on crotch rockets are usually wearing helmets. Older bikers on big bikes are not wearing helmets.