How do the gas station men take advantage of Dr. Spivey?

When the men stop for gas, the service-station man asks if they are from the asylum. The doctor tells him that they are a work crew, not inmates. The service-station man behaves rudely to the men and tries to charge Spivey more but McMurphy tells them that they are in fact criminally insane men from the asylum and are entitled to a government-sponsored discount. Harding perceives that mental illness has the aspect of power: the more insane a man is, the more powerful he can become.