It seems like the great majority of posters here are *not* suggesting it should be mandatory, PC. Just that it's a pretty darn good idea.
There are certainly arguable reasons for saying that people have an interest in the safety of what others do, however. The reason insurance can cover the bills from injury is that ALL of us pay in that money. Those who are incapacitated by injuries, OTHER people are the ones having to take care of them for, potentially, the rest of their lives. Etc etc.
Thus there are, and have always been, plenty of laws on the books that restrict a person's freedom to do whatever dangerous thing they want, or require you to take precautions to reduce injury.
Now, me, I am not sure that adults wearing a helmet when riding should necessarily be in that category... but it would certainly not be without precedent, and there are non-insane arguements for that sort of law, even if you do not personally, on the whole of it, agree.
The only thing that bugs me is to see people paint helmet-wearing as pointless, ridiculous, or arduous-and-unpleasant, which it is NONE of those.
Pat