Light turns green, you are turning left. You pull into the intersection and wait for the oncoming cars who are going straight. Light turns yellow, but rather than hitting the breaks the oncoming idiots are accelerating to make the light. Light turns red, you are still in the intersection. It's happened to everyone, I'm sure including you, many times. Would justice have been served if you got a ticket, several hundred $ fine, and 3 years of jacked insurance rates for every time that has happened? Or you could have gone to court to put your word up against a photograph of your car sitting in an intersection (nevermind that the camera nor the court have any idea who was actually driving).
Also, there have been many cases of yellow lights being shortened to increase violations and fines (revenue) at red light cameras. So no, you don't always have plenty of warning.