So how do we decide which risk categories? and who gets to decide?
By any measure of societal impact bicycle head injuries is a small contributor, and includes a large percentage of helmet wearers anyway. So the societal benefit of a helmet mandate isn't that big.
But there are areas with much higher societal cost, such as unsafe sex or diet/obesity/diabetes? Since we're talking about societal cost impact, wouldn't it make sense to start where the impact is greatest? And if we want to limit it to bicycle accident/injury rates, it might make sense (to some people) to tackle the issue head on by removing bicycles from roads altogether?
There's more to life than worrying about who's costing you money somehow, and it doesn't take a rugged individualist to see that.