I tend to think this kind of thing… rule zero, as it’s commonly called… gets overemphasized and/or overstated. Most of the time, in the games with which I’m familiar, it’s not like a carte blanche for the GM to just do whatever they like. It’s more permission to change things to suit a particular desire or to ignore an absurd result of the rules.
And I think that there’s an implication of approval from the group.
A GM who’s just willy-nilly changing rules will likely find himself without players.
All that said, my previous statement was a bit inaccurate. I don’t think all rules do what I said. Obviously, “the GM gets to say” is a rule. I should have said that I think it is better when the rules prevent any one participant from having too much influence over the way the game goes.