I do agree with your argument, but helmets helps with more than just car vs bike accidents. People have an uncanny tendency to fall and injure themselves without the help of a car.
Groningen and the Nederland dont have any hills to speak off, but when you get to high speeds on a bike its good to be protected when you fall.
This law protects the biker against any trauma to the head while biking. Its is very easy to wear a helmet and a great way to be a role model for younger people.