As I recall reading somewhere, if you look in helmet vs no helmet states, the death rate is similar, but the cause is different. Obviously in no-helmet states it is head injury, but in helmet states, the causes result from excess risks being taken. Having a helmet, in short, gives the rider a sense of invincibility, which leads to greater risks taken (excessive speed, stunts, aggressive riding).
So the difference then becomes the circumstances in these deaths, and what we find as a society to be palatable. Take for example:
1- Rider without a helmet, going 40kph through a neighbourhood. This person is a cautious, aware, and law-abiding rider. A child runs out into the street, the rider swerves, hits the curb falls off hits his head, and dies (or becomes a veg).
2- Rider with helmet on doing the Lawrence Ramps at 120kph wipes out, wraps around a guardrail post, broken back, death or veg results.
I'd be willing to bet that example #1 is less acceptable to people as it was so easily preventable and was following all the rules, whereas #2 was being a fool and breaking all the rules. "But supr," you will say, "Rider #2 in your example was an idiot regardless of if a helmet was worn, so what is the point?" Well, the point is rider #1 also took an unnecessary risk. They are both equally fools (both are dead, right?), but as a society we cannot accept it when a "good" person dies, so we put laws into place to make us all feel good. Nannyism? Arguable, because this is not really wrapping your kids in foam rubber, as much as it is a social consciousness to protect good people from themselves.
Sure death costs less to all insurers and caregivers, but the real point is that it hurts more when you cannot put the blame on something else. If anyone here has had a close friend or loved one die in an accident, almost everyone at some points does the "if only I could have done more". In having laws such as those for helmets, and seat belts, that becomes our collective "more". It is really only when you realize the outcome was beyond your control, that you accept the inevitable result. Having laws such as these allow us to reach that end more quickly, because we realize that person's personal choice to break the rules was just that, their choice.