it's likely that helmets are worn in greater numbers in areas that are unsafe for riders (ie: NYC vs most of Europe) so of course more incidents will happen in areas where helmets are worn. correlation, not cause.

also, maybe helmets don't help prevent head injuries, but i still haven't ever heard a good reason why i SHOULDN'T wear one (aside from the weird part it's rubbing in my hairline) so i'll wear one until i do.