Why would Bungie do this? Aren't they all MS-hating Mac users?

Bungie in the past may have capitalized on their position in the smaller Macintosh market to play themselves as the underdog, but business is business. Microsoft offered to make Bungie the premier developers on their new Xbox gaming console, with a large degree of influence over how that platform would develop. In addition to that, being part of Microsoft offers benefits in terms of resources and stability that Bungie would be unlikely to obtain independently.

As for hating Microsoft, Bungie has stated that their objective all along was to make great games, without preference for or disdain against any particular computing platform.