Actually its kinda smart. Limited shelf space and a small user base means marketing OSX games in brick and mortar stores is an expensive proposition. However, if developers can bypass the stores, Steam can offer a greater profit margin, even if the amount of available games is limited. They can also sell OSX games that aren't in stores anymore, thus squeezing additional profit out of them. This could also stir up some Indie developer support if they have an avenue for distribution.
It won't bring any real competition to PC gaming, but for Apple users who want a few more options than what the stores offer it's a great thing. Kudos to Valve/Steam