Because those apps are part of windows. Tell me, what happens when you click Help > About (well, File > About now it's got the ribbon) in paint? I'll tell you... it comes up with a version info box for Windows 8... because they're part of the OS. The metro apps are merely pre-installed store apps, entirely separate from Windows and just pre-installed for convenience. The main reason being that they can be updated through the app update mechanism without requiring an windows update based OS patch.
Ok, I'll find something else. Say, Windows Media Player. Is also installed for convenience in some
versions of Windows (EC has even ruled that it isn't part of OS), receives out-of-band updates, can be installed and removed at leisure, has version separate from OS version. Or maybe Metro - ads, Desktop - no ads, because Desktop doesn't have an ad platform cleverly integrated?
In short, stop pulling wool over eyes with what is part of OS and what isn't. Is it authored by Microsoft? Is it installed from Windows DVD without me being able to choose if I want them or not? Then it's part of OS. Microsoft wants to cash in on people's gullibility and should be sued by EC for doing so.