Don't the Planeswalkers in Commander 2014 that say they can be your Commander contradict the fact the Commanders have to be legendary creatures?

Question

Ob Nixilis of the Black Oath and the other Commanders in Commander 2014 say they can be your Commander. Doesn't that contradict the rule that Commanders can only be Legendary Creatures?

Answer 1

The very first of Magic's Golden Rules:

101.1. Whenever a card’s text directly contradicts these rules, the card takes precedence. The card overrides only the rule that applies to that specific situation. The only exception is that a player can concede the game at any time (see rule 104.3a).

So yes, they contradict, and it's fine. And this isn't really unusual; tons and tons of cards have text that effectively says "now the rules work a little differently, and here's how." For an extreme example, the rules say you lose the game when you're reduced to 0 or less life, but Platinum Angel says you can't lose the game, and it takes precedence.

Answer 2

[Ob Nixilis wasn't Legendary at the time this answer was written, but every Planeswalker was subsequently made Legendary. The question and answers are still pertinent because Ob Nixilis is still not a creature.]

Yes^[1]. Commanders are normally Legendary Creatures, and Ob Nixilis is neither Legendary nor a Creature.

903.1. In the Commander variant, each deck is led by a legendary creature designated as that deck’s commander. [...]

But that's fine because of the first Golden Rule of Magic:

101.1. Whenever a card’s text directly contradicts these rules, the card takes precedence. The card overrides only the rule that applies to that specific situation. The only exception is that a player can concede the game at any time (see rule 104.3a).

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1. If it didn't, there would be no reason to put that ability on the card.