The key argument here is the same as Microsoft's previous issues with undocumented APIs. I think it's fairly clear that Microsoft wants Windows RT to be an iOS and Android competitor, not "Windows 8 on ARM". If you subtract the Classic mode from Windows RT it's in the same OS category as iOS, they just took a different approach to get there by basing it on their desktop OS.

The problem is that Microsoft has turned pretty much the entire Windows API into an undocumented API (AFAIK, please correct me if I'm misunderstanding the technical details here) and completely locked it down so that they're the only ones that can use it. It's not necessarily simple to move a large program like IE or Office over to a new architecture, but it's going to be an order of magnitude easier to do so when you don't have to rewrite it to an entirely new API (in this case Metro) while you're at it. Yes, Firefox can still put out a Metro version, but I would imagine that it would be a significantly greater undertaking than if they had the ability to write to Classic.

The unfortunate thing here is that this isn't necessarily an underhanded plot by Microsoft. There are definitely some benefits to the more sandboxed-style apps pushed by iOS and Android vs. what you can do with a desktop OS, especially when it comes to non-powerusers that just want stuff to work. From that point of view the Classic mode in Windows RT really shouldn't even be there at all, but if you want something like Office on ARM anytime soon then it's probably the only option. The trouble is that Microsoft wants to provide the additional benefits of a few key Classic applications while also trying to avoid having the OS turn into a bunch of poorly-ported desktop applications and I'm not sure they can have it both ways.