They do not have to disallow anything to make Ubuntu more consistent: Apple does not disallow anything from being installed on OS X (even now that the Mac App Store exists) and yet OS X is a lot more consistent than any currently existing Linux distro.
I don't think you can compare Mac OSX to Ubuntu in this way. The mac developer culture is different from the linux developer culture, due to the strong presence of the original UI design and human interface guidelines. When something doesn't match the OSX experience, its easy to notice- because you have that strong baseline, and there's a strong motivation to write software that matches that baseline experience. software that runs in OSX's x11 is obviously not as good as "real" os x software. There is no base for linux, so any widget set or toolkit is just as good as another, and there's no strong reason to pick one or another language, or "platform" or library. You're invited to install this runtime or that scripting language to get arbitrary programs to run. And linux people like it that way. That's what freedom looks like.
