a big part of a socialized health care system is the socialization of risk - which gives the majority certain rights to enforce specific rules, like helmet-wearing, in order to try and reduce costs. it also gives the majority the right to be coercive about unhealthy behaviour, like smoking and overeating.

it is eminently reasonable to transfer that risk back to the individual if they refuse to abide by those rules, for whatever silly reason they'd like. and, that applies to smokers and fat people, too.

let me put it to you this way: i don't think "it messes my hair" is a less valid reason than "because my religion says so". but, in context, that is the choice the individual gets to make: do they want to socialize the risk, and follow those rules, or take it back, in exchange for flaunting them? and, it doesn't matter what the argument is.

in the circumstance of being denied coverage, the patient would be able to argue (in court) that wearing a helmet wouldn't have helped, anyways. and, a judge could weigh that out.