Well there's also special code that caters for Android and Apple store development, so why not also Windows store. As long as there is choice, there's no problem. If the MS fork were to disallow any other target platform, that would be a problem. Otherwise I expect it to make it to the main branch soon.

I think you could make the case that people should not be locked in to UWP if they wish to develop an UWP app with Unreal Engine, so giving them an easy way to deploy on UWP, Mac, Linux, Android and iOS is beneficial to the engine ecosystem as a whole, even if you think that UWP is not something you should develop for.

the beauty of open source though will let people develop UWP games using epic's engine against its founder's wish.