Not all cost is of monetary nature. Wearing a helmet costs you comfort, agility, and time. That's why we only do it when the risks outweigh the costs. Which is

Not all cost is of monetary nature. Wearing a helmet costs you comfort, agility, and time. That's why we only do it when the risks outweigh the costs. Which is exactly the point: sometimes the risks outweigh the costs of prevention, sometimes they do not. Thus, this was an excellent example.