I think this is important to Microsoft, and actually quite a smart move. By letting other people use the Microsoft/Windows Store app natively built into windows

I think this is important to Microsoft, and actually quite a smart move. By letting other people use the Microsoft/Windows Store app natively built into windows, with their own billing infrastructure, they basically kill off any reason to have other applications for buying/installing software. Imagine if Steam allowed Blizzard/Activision/EA to sell their games on the Steam store, and as long as they pay their own hosting/billing infrastructure costs, Valve takes no cut? I think that would immediately kill the need for EA/Ubisoft/Blizzard/etc. to ever need their own applications to sell software.