Microsoft's announcement says "You can’t use Bitcoin to purchase Microsoft products and services directly at this time." It's not clear exactly what you can purchase. In-game items?
You basically buy credit to buy apps, games, dlc, music, etc. This a bit of relic from the days of Microsoft points that you needed to use on the Xbox 360. When I buy an app with my Windows phone it can directly charge my provider.
Which now includes Microsoft.
If you didn't know, Microsoft has chosen to accept bitcoin through their payment provider (otherwise that option would not appear). So I guess I don't understand what your point is.