The problem is that third party applications can cause security and performance issues and it's Microsoft who ultimately takes the blame for it. They exempt their own browser because it's their own browser.
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.
