This article from TorrentFreak goes into the detail, but to summarize, the pirates are getting vindictive.
If you pirate a game, you should not be surprised if the company takes some action against you, such as permanently banning you from XBox Live, in response.
Even though every Halo game has been pirated, this time some people have actively taken to trying to spoil the story for the rest of us. Choosing to pirate a game is an active act, and one should not be surprised if you might find comments on a pirate post that might give the story away. But posting spoilers in various threads, putting up a YouTube video of the game's ending is going beyond the pale.
If you do not want to spoil the story, be careful what you search before the release.