I no longer have a windows pc but when i had my Surface on W10 I installed the Kindle app. I was horrified by the lack of functionality and how it didn't even work for personal documents (which most of my books are).
I guess if they can't be bothered to develop the app this is the route they have chosen to go down. It doesn't bode well for microsoft's idea of apps for windows 10, especially if other companies follow the same path.
NB: Kindle for PC on my MAC doesn't support Personal Documents either.