Really? I need to read the legalese? Check
App Developer Agreement (Windows) - this is the full text of the App Developer Agreement an app publisher makes with Microsoft.
In section 3(a), it clearly states, "You, not Microsoft, will license the right to install and use each app to customers." In summary, it further states that if you do not provide a specific license to the end-user, that which is presented as Exhibit A on that page will be enforced.
Exhibit A clearly states, "BY DOWNLOADING OR USING THE APPLICATION, OR ATTEMPTING TO DO ANY OF THESE, YOU ACCEPT THESE TERMS. IF YOU DO NOT ACCEPT THEM, YOU HAVE NO RIGHT TO AND MUST NOT DOWNLOAD OR USE THE APPLICATION. " It also says, " The application is licensed, not sold. This agreement only gives you some rights to use the application."
It ain't Microsoft folks. It's the app publisher. All right there, all black and white. You don't own a single application you download from the Store, unless you wrote it. Hell, I'm pretty sure you don't even own the Windows Phone OS on your phone; I'm pretty sure Microsoft licenses - not sells - every piece of software they produce.
EDIT: Note I am not talking about Microsoft's ability to pull an app from the store for legal and/or technical reasons. Of course they're going to have the ability to do that, and if they do it for technical or legal reasons I would expect that the app shouldn't be made available after that.