My guess is they're trying to make a different format that will hinder people from modding their boxes and playing backups by making it impossible to play higher wave games after they are patched. I don't know how successful they will be though.
@SylverReZ, I also heard one of the Prince of Persia games was so unfinished that it required the "24/7 online" drm so a puzzle could be done and the game could be finished. And that when the Ubisoft servers were closed the (cracked) game was impossible to finish or something like that