The point being missed is IE is the only program that can wrap Win32 with a metro UI. Only IE gets to maintain a single rendering engine/ code-base. Chrome/ firefox have to maintain a Win32 and WinRT version.
That's because Metro is supposed to run lean as a tablet experience so they don't want every Tom, Dick and Harry just wrapping their windows apps to run on it. Chrome/Firefox/etc. already have Windows apps so now they have to create an app for Metro. No different if they were to create a Firefox/Chrome for Iphone vs MacOS.