Blame the bmx movement for making it "cool" to not wear helmets. Or just stupid bikers in general. I can't imagine the thought process that makes you think it's a good idea to jump a pretty large set of stairs (with NO transition and concrete at the bottom) on a crappy bike, with little to no bike handling skills (watch how he rides down that hill...), without even clearing the way, AND with no helmet.
I think everyone should ride with a hammer in their bag. When someone makes fun of them for wearing a helmet, give 'em a good whack in the skull. It'll give you plenty of time to get away, and when they wake up they might have learned something. Hell, you may just have saved their life!