First, there's security. Microsoft isn't banning browser per se, it is limiting access to APIs that might be insecure and could be used for hacking the system. Seeing how infested Android devices are with malware, this is a very good decision from Microsoft and doesn't differ any from Android or iOS (which don't allow standard Windows programs at all).
Secondly, if Microsoft allowed this, then Mozilla would just recompile their browser for ARM. They would not consider the limitations of the devices nor would they make Firefox use less resources. Seeing how bloat Firefox already is, this is a good thing.
Just like Mozilla states, Microsoft does not restrict them from making a metro-compatible browser. As you may know, Metro apps need to be done explicitly Metro in mind. It has different APIs from standard Windows APIs and is much more secured. As Metro UI is the main UI used on devices like tablets, this is a good thing.