It's misleading to imply that cyclists are (hypocritically) exposing pedestrians to a level of danger for which they would themselves want to wear a helmet. The level of danger of head injury that a cyclist or a nearby pedestrian is exposed to is below the level at which most people would feel a helmet was appropriate. (Notably, it's many times lower than the level that car drivers and passengers - people who do not normally feel a need to wear helmets - are exposed to).