Just as I suspected.
The same deal’s happening all over again, as the only way to get into the Halo: Reach beta is through the Halo 3: ODST disc.
Microsoft knows that having beta access exclusive to a retail disc is a good thing for sales, and that’s one of the reasons the Reach beta was exclusive to ODST.
Microsoft’s director of Xbox and Entertainment, Stephen McGill, had this to say about the exclusive beta:
McGill went on to say:
May 3rd is coming quickly, so if you want to get your hands on the beta when it hits, it’s decision-making time.
Source - CVG
