Actually I'll bet that even the Guardians of the Universe have restrictions on time travel. Time travel is very unpredictable because if you end up at the wrong place at the wrong time (No pun intended) you could irreparably damage the timestream and potentially cause a universe-ending paradox. Here's another example, Saving a species that co-exists with another species from annihilation could cause a future in which the race that was supposed to die out could become enslaved by the race that is supposed to inherit the planet. The moral of these two time travel examples is that even with the intention of making things a million times better, there's a higher chance you could make things a million times worse.
While I agree with you about the restrictions, hero's do it anyway and often! Also I beileve in the more robust universe theory where time being changed does not neccesarily ever lead to a paradox, more so the subject and device that went back have already been extradited from the time stream they originated from and therefore can change time freely, time in then changed for everyone for good or ill.