According to Valve, it was Microsoft's decision to charge for the Xbox 360 version of Left 4 Dead's soon-to-be-released DLC dubbed "Crash Course." In a talk with Eurogamer, Valve's Chet Faliszek said that Valve owns its platform, Steam, and Microsoft owns theirs. "They wanted to make sure there's an economy of value there," Faliszek said.
Typically, Valve makes downloadable content available free, and Redmond will be charging 560 Microsoft Points for Crash Course -- or about $7. Faliszek went on to conclude that while Microsoft handed out the first Left 4 Dead DLC free, it might be unfair to everyone else. Valve's upcoming Team Fortress 2 DLC will also have a price on it.
Last year, Valve's president Gabe Newell said, "On the consoles, they want us to charge money for [DLC], because that's in their model, and our model is very much more to grow the community by giving out free updates." While I can understand charging for DLC -- $7 seems a bit steep for a bit more content which has little overhead to distribute.