Police officers are some of the ding dongs that don't know the law either because it is so inbred here.
Most states law allows you to enter the intersection when the light is yellow, so if you are already in the intersection either before it turns yellow and while it is yellow, it is legal. It is not legal to enter an intersection where traffic has stopped and you can't exit the intersection because of traffic in front of you, but you can finish your turn after the light turns red so long as you can exit the intersection and you entered the intersection either before or while it was yellow.