So if you were a parent, would you tell your children, "Wear a seatbelt if you want, but if you don't, it's only you who'll get hurt"? The law reflects the fact that some people in the country do give a damn about others and that the government wants motorcyclists to be safe. It works like this:
Motorcyclist doesn't wear helmet, endangering self
Motorcyclist receives ticket which he doesn't want
Motorcyclist, wanting no more tickets, starts wearing a helmet
Through this law, this person has become more likely to survive in a crash - a desirable outcome, no?