Microsoft is trying to avoid UA-based blocking (e.g. Google once used this do block the Google Maps web app in Windows Phones) and styling (some sites only send the mobile version to webkit mobile browsers). Microsoft effectively masks its browser by changing the UA string to something which is detected as mobile Webkit, and this is (IMHO) wrong. Even if the site is "functional", a site owner should be able to opt to not support Windows Phones.

Suppose you are responsible for a site which carries a heavy and valuable brand. Suppose you don't have/don't want to allocate the resources needed to correctly develop and test your site in Windows Phone. You don't know how the site will run: you won't know about bugs, you can't guarantee a solid user experience and you can't guarantee performance. Ergo, you might affect the brand you carry. Although not ideal, I think it is valid for someone to say "I want to block my site in some device/OS, because I can't guarantee the quality and it might be bad for my brand".

I concede (and believe) that the end user should be able to circumvent these mechanisms by, e.g., changing the UA string. However, I don't agree with Microsoft approach of masquerading, by default, as Android and/or iOS browsers. Pardon my rant, but it's just Microsoft being Microsoft: bending standards at will and playing desperate when not it control of a given market.