This brings up the greater debate of, can the government force us to do something that is good for us, or are laws 'just' when they protect us against our will. On one hand we have the rights of the individual that need to be protected(the right to choose whether or not you want to wear a helmet) and on the other we have the duty of the government to protect the people(by requiring helmets to be worn).
This reminds me of the arguments between republican government and pure democratic governments. The reason that most governments today are republics and not democracies(such as that of ancient Greece) is that elected representatives are often better at making decisions for people than the people themselves. In a republic the government not only protects the rights of the people but makes decisions for them. These decisions are made daily by the appropriately qualified people, like economic policies or military doctrine, and are far beyond the abilities of most US citizens individually. This alone could cause one to say that the government has the right to make it law based solely on this principle.
There are few comparisons that can be made with helmet laws since they are unique in that without them, no one else except the rider will be in danger. Opponents to helmet laws would cite this as a prime reason to get rid of those laws in the interest of protecting the right to choose. But this 'free-market' style of approach, I feel, is inadequate to even warn people of the higher death chance of riding without a helmet. The end choice I feel comes down to allowing people to put their lives in danger or stepping in to enforce helmet laws as the will of a society that doesn't want, or is actively harmed by, motorcycle helmetless deaths.
Edited for spelling
Addition: The side effects of motorcyle deaths due to having no helmet may be a prime motivating factor.