Of course it does. But presumably most people who bike will never have a potentially fatal brain injuring accident, while having a healthier, more environmentally friendly, cheaper life (gas is expensive!), and on balance, it's a net positive expected benefit, for some set of plausible weights of pros and cons.
Ah, but this is the tricky moral question, do we enact a law that may decrease a large population's quality of life significantly, or do we enact one that doesn't, but allows for the death of a few?
Arguably, the greater good is done by not enforcing helmet laws, because vastly more people will bike, and thus get more exercise, outweighing the few major accidents per year.
> do we enact a law that may decrease a large population's quality of life significantly
Before asking that question, I wonder how many people don't bike because of the helmet versus how many say that they don't bike because of the helmet but really would take any excuse.