The point of helmets is to mitigate injuries in the case of an accident, not to prevent accidents from happening. I don't think a single accident has been prevented by seat belts or motorcycle helmets, but those are mandatory and subject to fines of $200 and $85. You also have to wear a helmet if you use a power-assisted bike or if you're under 16 and riding a bike. It doesn't make much sense why we don't have a similar requirement for regular adult bike users.
Actually, there are a host of good reasons why laws mandating bicycle helmet use are a terrible idea.