i don't think anybody should be forced to wear a helmet. i don't wear one while bicycling, but i might have different opinion about a motorcycle.
i just want to introduce a caveat: in exchange for their recklessness, they should be liable for their own medical costs, in case of emergency. i don't want to suggest denial of service, i believe in universality. but, that should be the price to pay: if they want to put themselves at greater risk of harm for no discernibly good reason, they should be forced to pay out of pocket for expenses.
and, that rule should be applied to non-sikhs as well. if you don't want to wear a helmet, and then get hit by a truck? you're liable for costs arising from it.
you should get due process, too.
but, that escapes this "religious accommodation" clause, and in the way that i think should be widely emulated. "because my religion says so" should not be a loophole in the law, or a way to evade following the rules. in many cases, those laws exist precisely because your religion says so, in the first place. rather, when something like this comes up, the rule should be struck down altogether - and the risk and liability should be transferred to the individual, instead.
https://globalnews.ca/news/4420093/sikhs-turban-helmets-ontario/?utm_source=Article&utm_medium=Outbrain&utm_campaign=2015