I could argue the specific point about motorcycle helmets but this isn't P&N. But, really, does the turban exemption actually make any difference to your life? (There _are_ possible situations where I'd object to special rules for the religious - this isn't one of them, it's inconcequential).
Edit - ha, as this now _is_ in P&N, I'll expand on the point. The motorbike helmet law is a law intended to protect people from their own behaviour, by limiting their freedom to take risks. As such its a much more tricky-to-justify law than one that is to prevent people harming others. And so it depends far more on consent. Such laws only really work if those subject to them are already half-inclined to undertake the mandated action (because they know it's for their own good, ultimately).
So it's a law that is likely to have problems if it comes up against a deeply-held religious belief.
I'm actually not that bothered about that particular law either way. Just as I don't care that much about compulsory seat-belt use by drivers.
But it's very different from giving religous-based exemptions to laws that are not about benefiting the very people whose freedom the law is reducing. I don't think religious beliefs should give you special rights to do things that directly affect others.