If you go read the EULA, you'll find you also pay a subscription. You did not purchase the game. You purchased a liscence to ACCESS the game. You have a digital liscence tied to either the disk and your account, or the digital file and your account, which grants you access to the game service. The EULA clearly states that this service will be changed to the discretion of Bungie, and that continued content purchase will be required to access all of the game.