The whole concept of "store" in Windows 10/UWP and Android is a pain. The default Microsoft/Google store gets trusted by default. On Android (at least on my phone) you can't set up the Amazon store and delegate trust to it (i.e. anything that Amazon says is Ok is Ok with me). You have to disable security to install apps from Amazon, which isn't great. Microsoft is doing the same thing, including the awkward side-loading option.
Windows store does have an "enterprise" option if you are going to use UWP for internal enterprise apps, but you still have to have Microsoft review, which also isn't great.
In my preferred universe, Microsoft and Google (and Apple, for that matter) would allow me to set up trust to any application source (store) I want (including, of source, Steam). If the current model of application protection were applied to browsers, every website would have to get SSL certificates issues individually for each OS, because there would be no mechanism to delegate trust to Verisign, Thwate, Entrust, etc. Users (and enterprises) should be able to manage trust for non-OS related applications and files, which means that we need a mechanism to trust third-party stores.