It's a necessary law. If time travel isn't regulated, whose to say what's a tragedy that needs altering? One person's massacre is another's righteous victory against a threatening force. If everyone were changing the timelines to suit their personal moral tastes, it would be utter chaos. Plus, even the worst of tragedies are a critical part in the chain of events that make up history. Mess with that and you're inviting all sorts of trouble. Say you stop one primitive civilization from wiping out another. And those survivors who should-not-have-been procreate. And those children in turn procreate, and 2000 years down the line one of their distant descendants, who was not supposed to exit, writes a philosophical tract, which splits ideologies, which provokes a war, which leads to a nuclear apocalypse, wiping out the entire species. Changing history is not as simple as picking a particular event you don't like and rewriting the ending.

[Narvin's voice was slowly rising through the speech, getting downright shrill by the end. He clears his throat and his voice drops back to his more standard smooth tone.]

And that's not even addressing the very real problem of people abusing time travel for more selfish ends, if such a law did not exist.