There is one quote from the GameSpot article that I found quite interesting which relates to a new agreement that was created in 2001 between publisher and developer.
"Among other concessions, Sierra agreed to relinquish intellectual property rights and to allow Valve certain rights to the online distribution of games."
Depending upon what these "certain rights" are then Valve could have an open and shut case. If, and I repeat if, they were given the rights to their own online distribution method then legally (I'm assuming of course cuz I'm no lawyer) they should be able to use Steam as an alternative distribution method (and make money from it). I'm sure Sierra at the time laughed at the thought of an online distribution method, especially for a game that is GB's in size, and didn't consider it much of a threat.
Of course, the devil is in the details, and unless you know those details, then no one really will know the outcome. Hopefully more details will be released in the future about this agreement.