Turns out head injuries by bicyclists are fairly rare but forcing people to wear bicycle helmets reduces the number of people bicycling quite dramatically which in turn means fewer health and environmental benefits. I'd be interested in seeing more studies as this is something that we have long know about motorcycle helmets. Pass a helmet law, fewer riders, fewer miles.
Maybe if this catches on for bicyclists, the nannies will stop telling us what to wear when we ride our motorcycles and just leave us the h3ll alone (which is all a libertarian really wants). I'm not holding my breath, though.