Which is the FSF's choice. The difference is that XEmacs has
decided not to bother about assignments and stuff, and due to
that decision they don't have such a general cooperation to
offer in a manner useful for Emacs. In contrast, the FSF
still has the choice to licence the Emacs manual under
different free licences because they bothered about the
assignment, and they bother about licences.
You seem to think that it is the duty of XEmacs to do whatever is necessary
to assure that GNU Emacs can use its code, even to the extent of hindering
the development of XEmacs itself. In fact, if we had insisted on such a
policy, we could not have gotten the sorts of corporate assistance (Sun,
Amdahl, INS Engineering and others) that we got.