Can see the reasoning behind this. Have been looking at Steam recently as I'm curious as to what would be available if I were to try some games on my PC. It is quite annoying to find the majority of the games I would consider are Windows-only and so won't work on my Mac (Batman, Skyrim, Mass Effect, Assassin's Creed).
Looking at Valve's games, such as Portal and Left 4 Dead, they all have Mac versions available. Well isn't that handy...