@langdonx: Why would you want to buy it on Steam? It requires a connection to Battle.net anyways and the image is clearly of a digital sale.
Simply because Steam has earned my trust in the digital purchase realm. Their servers are fast, their prices are great, their offerings are ample (overlay, friends, groups, screen shots, etc), and they've never blocked me from re-downloading content that I've purchased on any machine I run Steam on.
GFWL games exist on Steam, and I now have a GFWL account because of that. I don't see why Blizzard and EA can't also put their games on Steam. There's something to be said about customer choice.
Anyway, if I do buy the game, it will be a physical copy.
For some same reason valve games aren't on Origin/battle.net. They are competing with each other.
GFWL failed incredibly on its own so by putting it on steam they try to get what they can.