Aye, this is a problem related to a lot of PSN titles (warhawk's EP, etc.). It also won't allow other accounts on the PS3 (such as a brothers, etc.) to access the game because it requires verification that it is on the user's PS3 that purchased it. Why it uses the account I don't know, the PS3 MAC or other info gained upon installation would be better suited I think, but ah well.
Brief explanation even shorter, it checks to make sure it's you, so it needs to be connected to a internet access point to play many downloaded games/expansions.