As a former and future rider, I always wear a full helmet, but people have the legal right to ride without one.
If we were going for maximum safety for transportation, uncaged motor transport would be prohibited.
It's also a legal requirement to wear helmets when riding a pushbike too, which is something far more controversial as there's research to suggest that the effect those laws have on reducing cycling participation has a more negative impact on visibility and safety of cyclists. But I don't think I've ever heard someone say that mandatory motorbike helmet laws are a bad thing.
There is research which shows that the trend towards higher vehicular weight is caused by drivers desire to be, or at least feel, safer. Yet this makes less economically fortunate drivers less safe, has adverse impact on the environment, and doesn't improve human welfare.
Why do we regulate helmets and not vehicular weight? It might have less to do with improving welfare and more to do with individual politics. I fear people have more of a tendency to want to be an authority over personal liberties than of unwarranted aggressions. For example, how long have people protested gay relationships, but allowed sexual harassment to persist?