Same logic as why they're shutting off billing over a month before the data is no longer saved and the servers are shut down. If they sold new copies/codes right up to 9/29, you'd risk people buying copies only to find they can't even register them. "Omg, I paid money, opened the box so I can't return it, and I can't play!?" *angry-head-shake-slobberfest*
Edit: Fixed a few things because I was too busy having an angry head shake slobberfest