The choice to wear or not wear a helmet is based on each persons risk tolerance. For some people the act of riding a bike at all is too dangerous, for others riding is an acceptable risk as long as certain precautions are taken.

I've always found it interesting that more people don't opt to wear helmets in a car, it would definitely cut down on head injuries in car accidents... but it's never brought up or discussed as an option... and many more people drive cars every day that ride bikes/motorcycles.