After quick investigation I realized there's already some games from Steam here, Arx Fatalis being one of them. I rule!
Steam is just a distribution platform like others, editors can decide to add their game on it, in addition, if they want to, to "traditional" selling (in stores) or any other way.
If you read carefully my post, I'm not really talking exclusively about Steam, but about Valve. Titles such as Half-Life 2 (a Valve game) are useable only by the way of steam.
You like Steam or not (I don't like btw), for Valve, it's a way to have complete control on their games. On THAT point, I don't think they would let another company distribute their games... especially if it's a DRM-free distribution.
Valve may distribute Arx Fatalis on Steam, they're not the game's publisher, it's no way a Valve game...
You "rules"?... well, if it can make you happy...