Very few accidents happen because the motorcycle ride wasn't wearing a helmet. The direct effect of not wearing a helmet is pretty much limited to the rider who wasn't wearing one.

Given the above, there is no "general welfare" societal aspect to forcing people to wear helmets. Thus, no Constitutional authority for such laws.

Worse, a traffic cop who is writing a helmet citation isn't hunting for drivers who are dangerous to others. They are just acting as tax collectors, rather than doing the job that we have been told they were hired to do.