It's work they did, regardless of how they choose to distribute it. How they want to get paid for it is completely up to them. So what if it is something they could have added to the main game disk? You could argue that about anything released later for a game... they could have just put off the release date and included more and more ad infinitum. Buy it or don't based on whether you think it's "worth it", but I don't see any problems with developers doing this if they so choose.