[DanielStraight]

This is addressed in the article.

"However, if potential cyclists see everyone else in their community wearing helmets while riding a bike, it communicates that biking is a dangerous activity that requires special protective gear."

One of the author's reasons for not wearing a helmet is the image that wearing a helmet projects. The personal decision to wear or not wear a helmet affects the perception of non-cyclists, which in turn affects the number of cyclists. Or at least so goes the theory.

As an aside, since a lot of people seem to not have noticed and assumed otherwise by default: The author is female.