Whether or not you wear a bicycle helmet may be a case of personal preference. Sometimes you have no choice, and sometimes you do.
Some people wear their helmet all the time no matter what.
If you are entering a race or an organized century you have to wear a helmet if you want to participate. We accept that. The promoter is taking on liability and we have to play by his or her rules. No law says you have to wear a helmet when riding a bicycle if you are an adult. It is a case of personal freedom.