It's about Windows and IE, not WinRT. Unless Microsoft is going to call their tablet OS branding WinRT? If it carries the Windows name and contains IE, it's a potential anti-trust concern to regulators.
They are calling it WinRT.
Of particular concern are the APIs that IE has access to which Microsoft is denying other browsers, including VirtualAlloc / HeapAlloc and friends, CreateNamedPipe, ConnectNamedPipe, DisconnectNamedPipe, CreateProcess, and various others.
These APIs allow for things like making memory executable, a prerequisite for building a JIT [just-in-time compiler]. Without a JIT, it will be impossible to build a modern browser. These APIs also allow for things like spawning additional processes, and communicating between them -- something we use to isolate plug-ins for security and stability purposes and other browsers, including IE, use to isolate tabs and windows for security and stability purposes.
First, Microsoft is limiting plug-ins to pretty much Flash for Metro. Nothing else will be allowed. Why Mozilla is crying fowl over this, I do not know, but Firefox isn't known to be the most speedy of browsers. I don't need ten billion Mozilla Firefox plug-ins junking up my tablet. Second, it wouldn't be too difficult to come up with a new system of tab isolation, something that is system independent.
Windows on Arm is a different OS from x64 Windows. You can't just simply port code over and expect it to work, changes need to be made. The only reason legacy code was moved over was for Office to use, and Explorer. Quite frankly it shouldn't even be there to begin with. WinRT should be strictly Metro, but that's not up to us.