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You don't have to use steam to play the game...
Once installed and patched, you can copy the file anywhere and play starting the game from the .exe in the folder without any interaction with steam.

Steam is only required to install the game if you get a box copy.
 
What the hell? Why has Steamworks been forced onto the hard copy?
 
because the retail hard copy doesnt generate enough income to justify its own distribution system for the DLC store, most likely
 
You could always buy it from Zavvi.ocm, you get it quite cheap, and only 1£ delivery cost all over the world.