we require contributors to the GNU project to (...) assign the copyright to us.
Nope: "When the developers of a program make it a GNU package, they can decide either to give the copyright to the FSF so it can enforce the GPL for the package, or else to keep the copyright as well as the responsibility for enforcing the GPL."
Nope: "When the developers of a program make it a GNU package, they can decide either to give the copyright to the FSF so it can enforce the GPL for the package, or else to keep the copyright as well as the responsibility for enforcing the GPL."
http://www.gnu.org/licenses/why-assign.en.html