I'm not sure it is, really. The world changing comes about because Superman chooses to no longer respect his father's wish that he "not interfere with the world of man" and he actively starts to fly so fast that he's able to turn back time.
I mean, isn't space/time to work in that kind of bizarre way? If you travel fast enough, you experience time at a different rate (that's the theory of relativity, no?) and therefore, if you fly one way and go forward in time, why not fly the otherway and go backward?
I mean, regardless of the science behind the feat, the idea is that Superman has already demonstrated this kind of power, and at climax, makes a conscious decision to perform this miracle.
It's a bit hokey, and it's a bit too 'pleasant', but I'm don't think it's deus ex machina. :?