I think not just US, also Switzerland among other places. But either way, Copyright was signed over, and that has real implications. Although, in this case I gather that initially the Copyright owner for the given project was Microsoft, then some other foundation, and now .NET foundation. So, it's not like the maintenaners signed over anything they had, really. As sad as it is, legally it has never been theirs. Good thing the license protects some freedoms.
