XNA is developed by Microsoft chaps and is fully supported by Microsoft themselves.
Define "supported". I don't see any support options. It doesn't even work with the proper version of Visual Studio. And let's face it, Visual Studio Express is only free if you don't value your time (or your sanity), because you can't use 3rd party plug-ins (like resharper) with it. therefore developing for XBOX 360 is FREE.
Just because you say it, doesn't make it true - read the EULA again, or alternatively get a lawyer to explain it to you.