So that's why Steam went with OSX after Apple's own app store was already in place? Your theory goes against the evidence.
Steam for OSX was announced on March 8, 2010 and released on May 12, 2010. The Mac App Store was announced on October 20, 2010 and released on January 6, 2011. So Apple's App Store was released several months later unless you are counting the iOS Store as something Valve would have been concerned about.