Okay, no wait, I understand a bit better now.
Steam takes 30% of the money from all sales through Steam. EA wants to sell DLC in-game, and that way, Steam doesn't get a cut at all. While it might seem greedy on Steam's part, look at it this way. Steam is going to host the content for DLC they don't get money for, meaning Steam loses out (very menial, I'll agree) costs on bandwidth.
Now, I personally don't think it's irrational to demand that a game bought from Steam, will need to have it's DLC bought through Steam either.