Because even though the market for Mac games is pretty much non-existent right now, Valve sees that there's potential to make a decent profit.
The Mac customer base is small but growing. If you go to macrumors.com, you'll see that the announcement generated a lot of interest. And Valve knows how many of their customers are using Steam though Boot Camp on a Mac. If they can convince large publishers like EA to get on board, they can lock up digital distribution of games on that platform. Plus they might want to get in now before Apple starts thinking about an iPhone like App Store for the Mac.
This comment was edited on Mar 9, 2010, 17:47.