Steam has set any principles at any point in time? O_o
Well, at least when it comes to profit. I think that is why they didn't want others handle the DLC for any games sold on Steam, giving up one of their profit channels to competitors without a fight.
Wasn't that the reason EA pulled and refused to sell some of its games in Steam anymore, because Valve didn't allow EA take care of the DLC sales?
That's what EA claims was the reason anyway. Of course a week or two after that it seems they already had an all new Origin with planned Origin exclusive titles ready to go, so the timing is suspect at best and just seems more like a convenient excuse.
Of course it makes sense that they want to keep DLC sales to themselves, they do a lot digital as well and they even had their own digital store.
As a Steam consumer though it's vastly preferable to be able to buy the DLC for the game you bought on Steam through Steam as well rather than go to Bioware and buy bioware points to be able to get the rest of the game.
For Steam profit wise it's also a lot more beneficial of course, in part just because of that, and I think more importantly it was from their move to F2P games, and the fact that more games were going the DLC route, which meant that in the long run, they could be selling new games dirtcheap through Steam, using Steam for bandwidth and support/patching, and reap the profits from the DLC sales that would follow. A business model like that simply doesn't make sense for Steam.