This week's Windows Weekly broadcast spends about 20 minutes (from 2:00 to just past 20:00) on the topic of Microsoft banning competing browsers from Windows on ARM.
Paul and Mary Joe do a really good job of explaining things. The one thing I'd like to highlight, and Paul touches on this, is that were talking about more than just IE on the classic desktop on Windows on ARM. We're also talking about being disadvantaged in the Metro environment. That's actually the larger of the two issues.
Internet Explorer is basically one browser with two front ends. The back-end is basically the same in classic and Metro and it has access to special Windows APIs that no other app on Metro has access to. That means that this is not just a problem of other browsers being excluded from the classic desktop on Windows on ARM, but other browsers are also at an extreme disadvantage on Metro compared to IE.