Among other thing, I drive a school bus from time to time. To my limited knowledge, only in NJ are passengers in a school bus required to have a seat belt. No seat belts for passengers in school buses makes sense from a statistical or scientific point of view. Statistically it is more important to be able to get out of a school bus quickly that the protection a seat belt offers in a big 'ole yellow school bus. But, School Bus Drivers are required to have a seat belt! Why? Is the driver more important that the kids?? The answer is that if the bus takes a bounce, the belt keeps the driver in the seat where all the controls are. So, safety devices are not EXCLUSIVELY about injury mitigation, but in part, sometimes safety devices increase the ability to control a vehicle when something bad happens.
Certainly, helmet laws are primarily directed protecting the brains of people who would choose to not wear helmets. As has been pointed out, helmet laws may only shift the point at which death occurs. This may be of benefit to society because more accidents occur at low velocity that at high speed (with more kinetic energy comes more injury and higher death rates).
But: The roads are built and maintained at expense to the people in general. Every EMT dispatch comes at cost. (Even when the EMTs are volunteer - Mrs. Chaplain is an Ambulance Driver volunteer). So, there is some measure of cost when those in minor accidents suffer injury because of inadequate protection.
And: Helmets also mitigate loss of control in the event of foreign object strikes. (pebbles, rocks, birds, big juicy bug). A strike to the face or the head can result in loss of control. Riding into rain or hail without a helmet!? Now the 2000 mile a year rider who only rides on days not too hot, not too cold, not too wet, etc. may not experience these things as frequently. But, anyone who uses bikes for transportation is going to eventually experience getting hit with something that without gear would cause an off or an accident. How many 'non-accidents' happen because a rider has a helmet? And, how many minor offs result in no injury because of a helmet (and no injury means no accident report). I had one of these minor offs myself. If I didn't have a modular helmet I would have at least broken my jaw, and likely would have needed a ride to the ER, treatment, etc. As it was, I had a minor soreness and no visible marks. So, I saved myself some out of pocket expense, but I also saved some Rescue Squad a dispatch, and I also saved my insurance risk pool the bigger dollar payout. The local PD did not have to write an accident report. The bike did not have to be towed. Traffic was not tied up on the road.
Perhaps people should have the freedom to do things that others think is stupid - as long as it does not affect others. But, if an individual does not take reasonable steps to mitigate risk to others, then we end up with silly laws.
If I wanted to mitigate risk completely I would not even get out of bed in the morning. I choose to ride. I will do reasonable things to mitigate risk of injury. I won't ride without a helmet (well maybe duck walk the bike back to the regular parking space after changing the oil). I want to protect my head as much as I can. And, I don't want to take a bird to the face and, loose control, and hurt or kill someone (be that me or some innocent bystander).
More than any other vehicle on the road the motorcycle and the motorcyclist are an integrated system. A motorcycle can not even remain upright without the control input from the rider. So, the question is, how much a part of the ride is the rider? Is a helmet law so much different than the DOT requiring cars to have a windshield to pass inspection? Or, is a helmet law a subtle form of tyranny?
Motorcyclists represent, sadly (maybe) a very small percentage of vehicle miles on US roads. We are statistically insignificant. So, as one who has sworn an oath to defend and protect the Constitution from all enemies foreign and domestic,... I believe I would prefer to error on the side of liberty. We should not need laws to make people do what they should do anyway. But, when the day is done, if not wearing a helmet causes injury (physical or financial) to others then the people have a right to pass a law that offers protection to "we the people".