Long answer: One of the conditions for contributing code to the GNU project is assigning copyright to the FSF. That means they're legally the sole copyright holders for all software they distribute. The official reason for this is so they can pursue GPL violations in court without involving the actual authors of the code. It also permits them to change the license on any of their software. Finally, there are various reasons they might be able to do this anyway even if they didn't have the copyright assignment rule, so long as the new restrictions didn't conflict with whatever license the original software was distributed under.