I doubt it can be blocked. With non-jailbroken iPhone, you can only get apps from App Store but with OS X you are not limited to Apple downloads. You can download and install any app you want from any site in the world. Steve could block it from Safari but there would be workarounds available very soon plus other browsers would support it anyway.
iPhone OS is controlled by Apple but OS X isn't and cannot be
OS X isn't controlled by Apple? And it won't be long until Apple introduces the iTunes-like app store repository for OS X.