Are you not allowed to enter the intersection and make a left turn as the light turns from yellow to red?
At many lights, yes. At a LOT of lights no, especially on boulevards. In some cases you have to go straight through the intersection and turn completely around and make a right. At others you have to make a right, turn around and go back through the intersection. If you get behind a big truck or a little old lady in a boat, you can waste a lot of time and gas just trying to get from A to B.