I notice that no one complains that there is no way to install Firefox onto a Chromebook. And Apple not only makes it hard to get a browser into its App Store but has admitted that other browsers won't run as well because of security decisions which only give Safari direct access to the hardware.
This is only an issue because we still remember the antitrust suit from a decade ago.
Thank you for calling out the fact that the argument for other browsers on a Windows device is solely an argument for the 'traditional' fight -- the argument people are supposed to have choice on a Windows device (which is what the EU and antitrust litigation was all about). However, you also correctly pointed out that on an Apple/iOS device, most users are content with Safari; on Android devices, Chrome. Is this good for the consumer? Maybe. Let's not forget that the consumer did choose to buy the device on which the browser runs.
In the modern day of compliant browsers, have we narrowed the need for replacing our preinstalled browser? On a Windows RT/ARM device, which is more like an Android tablet or Apple iPad, Microsoft is trying to vie for performance -- battery and usability -- which has been the sticking point for a fair amount of complaints. Who's complaining that Android devices don't run other browser, or in your case, that an iPad doesn't run another browser well (if at all)?
This is plain-old-FUD being propagated by Google (Chrome) and Mozilla (Firefox). Nothing changes on the Windows 8 desktop. You can still get any browser you want.
Google even just released a beta of its Chrome browser specifically for Windows 8. It works on the Desktop just as well as the current Windows 7 version and it even has a half-baked Metro interface for this new Windows 8 browser. I say half-baked because it is not available from the Microsoft Store, nor does it follow any Metro programming practices (which was the whole point of Metro in the first place -- a uniform interface so you don't need to hunt for features). In short, you cannot install it without the Windows 8 Desktop as a backdrop.
The point is that Windows-RT is ARM -- not x86. So even though the Windows-RT Desktop looks just like the Windows 8 Desktop, it is NOT the Windows 8 desktop. Legacy code written for the Windows 8 desktop should not be expected to run in the Windows-RT Desktop.
This is really obvious since Microsoft has said outright that you will need to install MS Office '15' under Windows-RT because legacy Office (version 2010 or below) will not run under Windows-RT. (By the way, MS Office 2010 runs perfectly well on the Windows 8 desktop.
Frankly, I think offering Windows-RT with a desktop environment (probably done for file transfer compatibility with Windows 8) causes more confusion than it addresses.
Google and Mozilla will be able to produce whatever browser they want for Windows-RT but, more likely than not, those browsers are going to have to be written for the ARM platform and sold through the MS Store. Apple has the same rules but Google and Mozilla don't seem to care about Apple.