One of the things that makes cyclists safer is having more other cyclists on the road. I'm happy to wear my helmet, but if others don't, I just want them on the road. In aggregate, we'll all be safer, even though some will choose not to wear helmets.

Specifically: Even those that choose not to wear helmets will be safer due to the increase in ridership. The comparison to seat-belts isn't appropriate because my not wearing a seat-belt in a car accident increases risk for others (specifically others in the same car as me).

There is also fairly conflicted evidence that helmets are effective. There is a fairly narrow treatment range where they work well. Be hit by something going faster or slower than that range, and they don't matter much. They are nothing like full-face motorcycle helmets.

There is also evidence (although not well replicated?) that drivers treat helmeted cyclists worse than unhelmeted ones.