I don't know what this is going to cost EA, but this may end up being a good thing. People hate Origin but more importantly, all their games are already on Steam, so they just continue to buy from Valve. This might get a few people to keep Origin installed on their computer and help the Origin network effect out. Up until now, EAs only strategy has been to make their big name franchises Origin exclusive. This will probably work better and may not end up costing them much.
They actually pulled most of their top titles off Steam when Origin launched. Mass effect, Dragon Age etc. If you already own them on Steam then they remain in your library but you can't buy them on there anymore.
Dragon Age 1 and Mass Effect 1 & 2 are still available on Steam. Interestingly, they pulled Crysis 2 then a few months later added Crysis 2 Maximum Edition.
For Dragon Age 2 and Mass Effect 3, I agree with their reasoning. Steam doesn't allow in-app purchases that circumvent their distribution system, and that's a load of crap.
steam isn't an open 'app store' though. It is entirely curated through valve. Valve chooses the games to list and if they aren't happy, they will not ask you to list with them. They could easily refuse to sell games if you did that.
Crysis 2 came back once no more DLC was going to be released - hence 'maximum edition'.
The no in-app purchases which don't cut in Steam was introduced when they began allowing free-to-play games on the service (which are usually monetised by microtransactions, and would be a money-loser for Valve if they didn't make a cut from them). I don't know why Steam can't make an exception for EA titles (or those purchased from the store rather than F2P), but that's the reasoning.
