A post to the 2K Forums explains why the just-released Sinclair Solutions Tester Pack DLC for BioShock 2 was already present on the game discs, saying this was to ensure multiplayer compatibility (thanks Shacknews). Word is:

I noticed there was a bit of confusion about our Sinclair Solutions Tester Pack file size, and I wanted to clear things up for you. The way our engine and game structure works is that people need to have the exact same content for people to play together. One of the challenges 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.

I know some of you have strong beliefs about DLC, and I'm not here to sway your opinion or convince you to buy our stuff - if you like what we're offering, I hope you get it and enjoy it. If it's not your speed, enjoy BioShock 2 as we released it.

I want to let you know that DLC is not interfering with our patching capabilities, which are a top priority and are still being actively worked on. We're committed to supporting you and making BioShock 2 the best experience possible.

I hope this clears up a lot of the questions and confusion in this thread!