Treyarch says that this will be their goal post release. To keep the fans engaged and playing the game so they will not want to trade it in. This makes sense, as game publishers hate it when you buy a used game so they want to stop used games from going on the shelves. Some, such as THQ even liken buying a used game to piracy and theft.
This is a better way of doing it though that EA’s online pass, which forces you to pay for all the games features. They are going to make you keep the game by actually supporting it! Let’s see if Treyarch can keep this promise though and they do not fall into the ever-growing rumor of subscription membership.