Actually its not true that only Metro apps will be sold via the store. All traditional applications based on XNA(which is C# based) & even C++ apps will be sold Via the store i.e. MS wont host the files(i.e. games in this case) but will provide a link to which to download from. This allows them to recieve the recognition most of them deserve but MS does not charge a cut as they are not hosting the files which only seems fair.
Also, by integrating Xbox Live service & UI(as an app) they could potentially open up XNA games to even more folks ( this is my guess & nothing I know has been announced along those lines)