While I can understand some people's assertion that Superman's power to turn back time, specifically in S2, is a cheap ending, I think that point of view overlooks something crucial; Superman's power to turn back time is NOT a fix-all because the next day, Lois has recollection of what hapened in the movie. Moreover, she conceives Jason in SR. Therefore, to say that Superman can just turn back time everytime someone dies is inaccurate. The ending of S2C plus SR shows that Superman cannot simply rely on turning back to undo something bad. I'm gonna go out on a limb here and say that even when he turns back in the ending of S:TM, all the events of the movie don't get undone because the missile that he threw into space still releases the villains from the phantom zone.