Red light cameras are legal in the United States because they are used to catch drivers running red lights. The purpose of a red light is to give vehicle occupants enough time to cross the white line before turning. Camera evidence can be difficult to challenge because it is usually clear whether or not an intersection was properly navigated at a RED traffic light.
In addition, automated traffic enforcement has been shown to reduce both crashes and injuries at intersections where cameras have been deployed. Reducing collisions saves lives and lowers insurance premiums, while better adherence to safe driving behavior reduces the need for police operations elsewhere on the road.