Bungie are true showman and even though the reality is their games aren't really all that inovative since the first halo, they're like vanilla but f@#$en great vanilla!
Vanilla wrapped in bacon, involved in a helicopter chase with ninjas on fire through a hospital zone (I think that was how halo2 was to be described).

Anyway the point of what I'm saying is that I don't think MS wanted to be upstaged by bungie as they're now not part of the fold anymore so to speak. I especially think Bungie were pissed off about being forced to pretend the 360 doesn't have a HDD, given the way they've developed the previous halo games that would have been a real thorn in their side.
I'm guessing that it may have been one of the reasons they decided to depart from MS's banner, we all know at least three third party games have relied on the 360 having a HDD, I wouldn't be surprised if we saw Bungie's next game do that also.