I think it has to do with
- Testability - it stems from the idea of keeping the Views thin ala the ModelViewController / MVP / MVVM patterns for building GUIs. That way the view just has controls which are bound to a backing presenter class, which can then be easily tested without having to involve the GUI. You can achieve a remarkable degree of confidence just by testing via the presenters. It is significantly faster than testing via the UI too.
- Moving to declarative programming as compared to imperative programming - XAML is declarative programming. You don't need to test XAML markup. Also it is MS code that is more or less guaranteed to work and remain working. So you can be quite certain that initializeComponent won't break with a check-in that you just made. So theoretically, you could get by without ever testing your views - provided your code-behind has no custom/hand-written logic.
