Something seems to be up with Activision's 007 games, as Blood Stone, Quantum of Solace, and the recently released 007 Legends all seem to have been removed from various online stores. The removal was first spotted on Steam by neogamr.net (via Joystiq), though a deeper look reveals that even Activision's own online store no longer sells the three games.
This only seems to apply for digitally distributed PC downloads, as you can still find physical copies of the games at your retailer of choice. As others have suggested, it's possible that Activision's rights to the 007 license lapsed, though that would be surprising given that 007 Legends was a fall 2012 release. We've reached out to Activision PR and will update this post when we have an official response.