I'd say it's more an exemption for their own rendering engine. IE10 is more than just a "browser" in Windows 8, it is the engine used to execute several Windows 8 applications.
It's an exemption for whatever is preinstalled on the device :) The actual technical mechanism behind this "ban" is simply that installing desktop apps in general is blocked on Windows RT. A "Metro style enabled desktop browser" (what Metro style IE and Metro style Firefox are) is really just a desktop executable that acts like a Metro style app.
