I don't think EA is being greedy at all.
It's not about Steam taking care of their customers. Steam customers could still get the DLC from inside the game client.
Instead, Steam/Valve want a cut of all the DLC revenue. Thus they have stipulated that you must be able to buy the DLC from Steam, rather than in the game itself.
All Dowload services should be able to sell the DLC for the games that are sold on it.