I hardly think Valve is the greedy one here otherwise why would they allow people to pay just a single one time fee for a game that has been giving people years of enjoyment. Also as stated below, I think more than a few people are assuming Sierra/VU funded the game which they didn't do (thus there is no return on investment required). Simply put I think Valve is fine with Sierra distributing the game on CD but they want their chance to explore the possibilities of doing their own online distribution via Steam and see how it goes. Sierra makes money off the CD and Valve makes money off the online distribution. If anyone is greedy here, it looks like Sierra to me. Valve paid for and created Steam on their own but it looks like Sierra wants a a cut (or most) of the profits for that distribution method even though they have nothing to do with it.
I guess what needs to be determined here was this a development/distribution agreement (which I don't think it was) or was it just a distribution agreement (for a specific medium)? If it was just a distribution agreement for Sierra to distribute the game via CD then I can't see how Valve is doing anything wrong with exploring their own distribution via Steam.