Why do motorcycle drivers not have to wear a helmet?

According to the American Motorcycle Association, a majority of states in the United States, including the District of Columbia, require both operators and passengers under a certain age, usually 18 to 21, and Novice riders, to wear a safety helmet that conforms to federal Motor Vehicle Safety Standards.

Operators and passengers over the age of 21 years may be exempt with insurance requirement stipulations.

However, Illinois and Iowa does not require a helmet for either adults or children.