Briefly, it's a classic case of "personal freedom" versus "public good." Helmets not only save lives (when it's life or death) but can prevent long-term injury such as traumatic brain injuries, which can be a drain on state resources if the person is not privately covered. So I'm guessing there is a cost/benefit analysis done in some places, weighted with "freedom" (to be an idiot) versus "public good." And that's why some states come to different conclusions (AZ, NH, RI, for example). It's an interesting question. When seat belts were made mandatory (after decades of studies showing they saved lives), there wasn't much of a push back from the population (but some from the auto industry). When wearing seat belts became mandatory, there was a little more push back from the population, but now it is accepted as standard. Helmets are similar, only you get more of a gung-ho "FREEDOM" response, I think, in some of those states that lean a little more libertarian. And, let's face it, it's hot as hell in AZ in the 115-degree heat. Didn't stop me from wearing a helmet when I lived there, however.