EA Claim it to be this:
Will ME3 be available on Steam?Strange way of spinning that they can't have there own 3rd party store and must use steam for DLC.
During initial release Mass Effect 3 will be available on Origin and a number of other 3rd party digital retailers, but not on Steam at this time. Steam has adopted a set of restrictive terms of service which limit how developers interact with customers to deliver patches and other downloadable content. We are intent on providing Mass Effect to players with the best possible experience no matter where they purchase or play their game, and are happy to partner with any download service that does not restrict our ability to connect directly with our consumers.
The funny thing is, I am more likely to buy DLC via steam than any other system. This is due to the fact that I already buy items via steam.
After the utter disaster that was Dragon Age 2 and Mass effect 2. I won't be buying another Bioware game. This is another rant in itself =p