The 2K Games offer is timed for the run-up to the launch of Civilization: Beyond Earth at the end of this week. While Civilization 5 is set on Earth and takes place during recorded human history, Beyond Earth is a speculative science fiction game set on an alien planet in the 27th century.
Polygon will be running a review for Beyond Earth on Thursday. 2K says the game is now available for pre-load. Meantime, that free Civ 5 Steam offer is open from now until Thursday.