It is rated for Language, also Blood and Violence.
So that wouldn't be a much of a reason to exclude it....
Maybe Bungie knew slapping on a M rating wouldn't keep the masses of 8 year-old's from getting their hands on it. Probably just covering all their bases for a lawsuit.
That's my best guess.
And all those moaning about someone "growing up" really outta just lighten up for a change. Who honestly hasn't used profanity in a gamertag at least once, if not a thousand times?