XNA is a framework for building games. Rather then spending 4 hours writing the code that outputs your stuff in DD 5.1, XNA has a function built in. There are things for outputting video/vector images to the screen, audio, controller, and all the other things that make creating a game a nightmare.
In short, XNA is intended to give developers the tools to make games, rather then humongous advanced programs that run games on top of themselves. It lets developers spend more time on making the game look good, and making the art assets rather then spending months reinventing the wheel.
Yes, for now only people with XNA will be able to run code, etc, but the point is obviously to allow more freedom in development and to streamline the process of making games in general...so of course for using the toolset and having ms handle sales and distribution will make them want to have a cut. Im sure sony would do the same
This isn't a greedy thing, this could actually turn out pretty damn good. We'll be supprised what we can figure out once we can start running our own code on the 360, even in a limited state with a hypervisor still watching.