A lot of people have been upset about that the Sinclair Solutions Tester Pack, the first multiplayer DLC for Bioshock 2, seemed like it was actually on the Bioshock 2 disc from the start. 2k Elizabeth, one of the community managers at 2k Marin, hit the forums to explain why, stating that the way their engine and game structure work is that "people need to have the exact same content for people to play together."
She also said that a challenge "with post launch content for MP is that it can split the player base, and we want to avoid that whenever possible. For this content, creating the DLC package the way we did allowed for us to not split the player base - so whether you purchase the new content or not, you can still play with your friends." (Thanks to Kotaku for spotting it.)