SourceBioshock 2 and its downloadable content packs are not currently available for purchase from digital distribution platforms, such as Steam, PlayStation Network Store, Xbox Store, GreenManGaming, the Apple Store and Amazon.
First pointed out on the PC Gaming subreddit, it is suspected that the game was pulled for sale from digital retailers due to music license issues. This follows the removal of BioShock 2, along with Far Cry 2, BioShock 2: Minerva's Den DLC, Crysis 2, and The Darkness, for sale from the Xbox 360 games store last month.
UPDATE
Polygon: 2K says the game will return to digital storefrontsUpdate: A representative for 2K says the game will be returning to digital stores soon. Their full statement follows.
"BioShock 2 has been temporarily removed from the Steam, Xbox Live, and PlayStation Network marketplaces and will return to these digital stores soon. We'll let you know when that happens. In the meantime, this in no way affects the single-player or multiplayer experience for those who currently own BioShock 2."