Why is there a need for a law? Someone should be free to ride without a helmet, whatever his religion, as long as he can find an insurer willing to provide adequate coverage (or be able to self-insure) for his medical bills.
I'm not fined for being fat, or even given the bill for my own (hypothetical) coronary, despite the enormous liability that gives the state. As long as that's true, I'd say motorcyclists can reasonably doubt that protecting the state is the real purpose.
In Canada, for example, healthcare is a shared burden across all people. Which alone is a strong argument in favour of safety enhancing policies.
There is also good argument for the role of government to help take care of its citizens. The danger of not wearing a helmet is so great that the government steps in to enforce people to do it. It doesn't matter how well the person says they are prepared for the risks or has "done research", the data are clear that they are objectively wrong to not wear one. Sure, people can say they are smart and ready, but time and time again we see that people make choices against their best interests. So, the government steps in.
Because we live in a society? Because suiciding yourself on public roads should be illegal?
On the other hand, if we believe that people should be allowed to hurt themselves if they can find someone to cover their medical bills, then the question becomes where do we draw the line? Riding a motorcycle without a helmet seems like a reasonable one. What about with a helmet? Motorcycles are inherently dangerous. But all road traffic is more dangerous than trains. Should people need a personal health insurance add-on to drive a car? What about eating unhealthy food?
Even if the line is drawn clearly, now only the rich can afford to do certain dangerous things and everyone else can't. Due to money shifting into the private sector, less and less will be covered by state insurance, making things that are allowed even today (like many sports) to be off-limits to anyone middle-class and lower. Now we've turned a situation where everyone has reasonable but equal limits to one where the rich have no limits and everyone else has unreasonably strict limits.