[dagw]

One theory. People who wear helmets tend to be people who or less "into" biking and more insecure about biking and their biking skills. Thus they play it safe and default to following the rules. People without helmets tend be the ones who view themselves as better bikers and feel more secure in their biking skills and thus think the rules and more like guidelines and that they, with their superior skills, know better and can safely ignore them when they feel like it.