Contracts are fairly easy to breach if you have a good lawyer, and I imagine Valve does.
As mentioned, this is the way of the future for many companies. When they're big enough it makes sense to distribute themselves. They can charge less and profit more. Of course, they can't charge less yet, but the day will come. Publishers will still be needed to fund smaller games, but they'll do online distro, too.
Someone also mentioned battle.net - it's entirely different and very cheap for Blizzard. All battle.net is is a chat service, essentially. Very little bandwith is used. When you play a game you're direct connected to whoever is hosting the game - someone else's computer, not battle.net. All blizzard does is host the chats and matchmaking as well as store basic statistics.