"I just tried on my other account and you are right, I cannot use the maps on the same Xbox with another gamertag even though it's all on the same xbox," notes forum user HalfericTuck, just one of many complaining fans. Reader Aaron, who alerted us to this issue, told Joystiq, "My girlfriend, whom I live with, cannot play the maps. My friend's teenage sons cannot play the maps."
This restriction does not square with the stated terms on the download. When users download the add-on, they are told, "You can use this item on the first Xbox 360 console you downloaded it to. Access to this item will also be granted to all users on this first console."
We've inquired with Activision regarding this issue, and will let you know when we've heard back.