In my imagination, there's a terrible accident that leaves someone other than the helmet-wearer paralyzed or dead, and investigators are surprised to see that one driver was wearing... a helmet?? It's almost like he knew he was going to get into an accident -- perhaps even intended to. Certainly, that's what people would think reading the articles about it. Perhaps a jury would, as well.

If this happens to you say you did it because of my blog post. If you pay transportation costs I will even testify for you at a trial.

anti-lock breaks are said to increase risky driving behavior, after all

Economists call this the "Peltzman effect" and it seems robust. It does reduce the social benefits of driving helmets. One economist took the implications of this effect to their logical conclusion and suggested that steering wheels should have a spike pointed at their drivers.