They're not necessary, they're just a very good idea. So good, in fact, that a lot of states make them compulsory when riding a motorbike.
But sometimes states decide that, even if they are a good idea, there are other things that take precedence. Even if that other thing is net detrimental to public health.
You can disagree with the decision. I happen to not think that religious rights should trump public health. But it's entirely reasonable to make exemptions when there is a conflict of priority.
And, in this case, there's probably enough evidence from other states that have this exemption to make a reasoned public health choice. The UK have had it since the 70s, for example.