JG sends a couple of very good links that explain why mandatory helmet laws are bad. Here and here. Besides all the usual safety issues, there are some secondary issues that are harder to explain. The first does a good job with this one:

Health effects aside, the real question here is whether or not the state believes that parents should have the primary responsibility for raising children. If it does, it has no business micromanaging something with a risk as low as riding a bicycle without a helmet. Parents must routinely balance risks that lawmakers never even think of. A school-hating child is perfectly capable of figuring out that “losing” his helmet at the last minute means that he can miss some school. A parent alert to the dodge would probably decide that making the point that losing one’s things is no excuse for missing school is more important than the (slight) increase in the risk of injury from riding a bicycle without a helmet. The child would be shoved out the door and off to school without a helmet. Should the state ticket or reward such discernment?

And now the medical journal JAMA reports that the World Health Organization (WHO) declares that, "Safety is a fundamental right. It is essential for the attainment of health, peace, justice, and well being." It goes on to explain how this should help health professionals to lobby for things like helmet laws. Soon, the United Nations will get involved.