For a country that's ostensibly headed down the path of deregulation, mandatory helmet laws in the US seem contradictory.
Go to almost any European city and observe the cyclists. Where I recently traveled in France and Spain, almost none of the cyclists were wearing helmets, or any of the other goofy cycling gear like reflective vests that many US cyclists advocate for. Traffic in the European cities is as bad as the traffic in any American city. Despite the traffic, my routine observation was that most French and Spanish motorists were very respectful of their cycling brethren, and drove with due caution in their vicinity.
In America, however, the motorists all behave like they own the road and the cyclists are in their way. The prevalent attitude in the US seems to be - wear a helmet so you don't hurt your head when I run you down.
IMO, mandatory helmet laws are total BS and are only one of the latest salvos in the 'blame-the-victim' mentality and behavior that's so common in the US. If US motorists and cyclists all took more responsibility for their own behavior and acted accordingly, there would be no need for mandatory helmet laws - helmet use would be a matter of choice among cyclists, and the motorists would drive way more responsibly, as befits the operators of potentially lethal motor vehicles.