It's very simple: Wearing a helmet may help in a tip-over, low or mid-speed crash, but it probably has no effect in a high-speed crash (where the rider is spattered like a bug, and no offense to anyone who knows people this happened to--it is tragic). Wearing a helmet can have a seriously detrimental effect if one is run-over (and drug due to the helmet catching a low point of the vehicle). A helmet usually causes additional neck fatigue, which for some people is problematic. Helmets impair vision and hearing to differing degrees. Helmets may save a life, but the quality of living may be "not worth it."
Most importantly, you'd be safer in a car, truck, or even more safe in a bus, but we don't outlaw bicycles, motorcycles, and cars because a bus is safer. It's personal choice.
There's the logical truth, safer is not always "better." Safer is a myth, as it's relative. Safer than what? It's not safer than being on a bus. Wearing a helmet on a bus could arguably make you "safer," but at what expense? "Safer" isn't always "better."
Driving without a helmet also has positive effects on society. The pleasure I have driving without a helmet is spread throughout the day to lots of people, as starting my day off with a beautiful ride into work at sunrise makes me happy, and I share that. I drove down to the Harley Davidson 50's Ride last night, there were five helmets that I saw out of about 1,200 bikes. (Surely there were more, but that's all I saw.) Well, two people I rode down with had helmets too, so seven...anyway, there were tons of bikes and at least 90% of the people didn't--there's a reason.
Soda has negative effects on society, as does a steady diet of "fast-food." People choose to do that in the inner-city, even when Michelle Obama had that "initiative" to force stores to sell fruit instead of candy bars--people wanted the candy and junk food.
So there you go, not wearing a helmet has pros and cons. So does skydiving, but skydiving is way more risky (and they do wear a helmet).