Gabe Newell ordered to make in-person deposition for Valve v. Wolfire Games lawsuit
(www.gamesindustry.biz)
Steam wants to keep it that way. Any references to other storefronts in your demo or game aren't allowed either. So if you're demo has a list of every place to buy the game, it's rejected, can only contain steam. Steam is deathly afraid of losing the advantage.