EVERY person that wants to build a third party YouTube app for their device is required to do so using HTML5. This is not a requirement only forced on Microsoft. It's one of the reasons why there is no native app for the Roku, and why the Apple removed their YouTube app from iOS.
Microsoft is bitching about the same thing when it comes to their Calendar app on Windows Phone and Windows 8. They claim Google cut them off from ActiveSync and Google tells them to use Caldav. Well, even the upcoming Windows 8.1 release still doesn't use caldav. Microsoft has no interest in adding Caldav support to Windows. Caldav directly competes with Exchange calendar functionality. At one point (or maybe they already did), Microsoft was going to yank IMAP support from Exchange server. Their solution to server side mail was Exchange Server and Outlook Client.
This really is a classic case of Microsoft not wanting to implement open standards, because they compete with their proprietary business protocols.
Running the Youtube site on Android or iOS gives you an experience almost as good as the native app. It really isn't Youtube's fault that Microsoft ships an inferior browser with their product.