It is not any more redundant than other rules. It sets the assumed baseline of authority distribution in the game. It is directly related to the rules and instructions that tell who is supposed to decide what, and how. That you don't like is nothing more than a personal preference.
My point is not about preference; it follows from the basic logic of play.
Settling on a technical set of rules to adjudicate a small-unit skirmish, for instance, is not trivial. Likewise a satisfactory set of rules for spell casting, or sneaking through catacombs, or anything else. Especially if the goal - which it clearly is, in D&D's case - is for those rules to have a certain crunch-y "presence" during play. (Cf Cthulhu Dark's rules, which would not satisfy typical D&D groups as best I can judge.)
But saying "You all have to agree on a setting" or "You all have to agree on allowed races" is tautological. Setting out a rule for it just reiterates a basic premise of getting the game moving. Purporting to allocate authority over that to one party, when the game can't get going unless everyone agrees, is misleading on top of it.
(Again, there are exceptions: formally structured play, say in a tournament or league. But is that who the D&D rules are written for?)
The point can be worked through in more detail, using as an example the 4e PHB, which describes the DM thus (p 8):
One person has a special role in a D&D game: the Dungeon Master (DM). The Dungeon Master presents the adventure and the challenges that the players try to overcome. Every D&D game needs a Dungeon Master - you can’t play without one.
The Dungeon Master has several functions in the game.
*Adventure Builder: The DM creates adventures (or selects premade adventures) for you and the other players to play through.
*Narrator: The DM sets the pace of the story and presents the various challenges and encounters the players must overcome.
*Monster Controller: The Dungeon Master controls the monsters and villains the player characters battle against, choosing their actions and rolling dice for their attacks.
*Referee: When it’s not clear what ought to happen next, the DM decides how to apply the rules and adjudicate the story.
The Dungeon Master controls the monsters and villains in the adventure . . . The DM’s job is to provide a framework for the whole group to enjoy an exciting adventure. That means challenging the player characters with interesting encounters and tests, keeping the game moving, and applying the rules fairly.
If we look at these, we can see how they might be varied or elaborated on. The second and third points (Narrator and Monster/NPC controller) are common to any RPG with a mainstream/traditional role allocation: not just D&D (all versions I'm familiar with) but Apocalypse World, Burning Wheel, Cthulhu Dark, Prince Valiant, HeroWars/Quest, etc.
The first admits of variation, because it doesn't tell us
who decides what the adventures are about. More typical in D&D is for the GM to do this. 4e includes the option for the players to do this, via player-authored quests. But either way, there is no "absolute power". Even a GM who does not take up the idea of player-authored quests is going to need to prepare material that players will find interesting. Otherwise the players will depart!
The fourth point sets out a referee function. Again, this is fairly typical in RPGs although more fraught in my experience in D&D, because of the complexity of D&D's interlocking rules components. The reference to "adjudicating the story" I take to be much the same as
@TwoSix's comment upthread about resolving uncertainty in the fiction.
Deciding how to apply the rules is trickier. In this job description, that is constrained by a pre-condition of uncertainty - so the rules are taken to be binding even on the GM, but there is a recognition that uncertainty can arise.
For the rules to try and push the matter further - for instance, by stipulating that the GM can suspend or ignore the rule at any time - would in my view be futile. First, that is really rewriting the rules - it makes the key rule "GM decides", and then the other rules become something like optional prompts or decision heuristics. And second (and even setting aside the oddity of saying "Here's hundreds of pages of rules that you can and should ignore if you want to" - what, then is the point or function of all those rules?), a rule like that simply can't bootstrap itself - a game which presents itself as a game, with multiple participants, can't then simply stipulate that one of those participants is the only actual player. Even if it tries to, the other participants will want to set their own degree of participation: they are turning up to play a game of shared imagination, and the rules on their own can't tell them that their imaginings are subordinate to another participant's.
Which is ultimately why I regard rule zero, at least in the form of "the GM has absolute power", as making no sense. It is an attempt to state that in a non-solitaire game, only one person is beyond doubt a player.
