If the DFSG is to be applied to documentation as well as to programs, why is the text of the GPL included in Debian, if it says that it cannot be modified at all?
Because the verbatim text of the license must be distributed with any work licensed under its terms. This is not specific to the GPL; almost all free licenses require that their text be included verbatim with the work. As a compromise, Debian distributes copies of the GPL and other licenses under which the components of Debian are covered. This compromise will not be extended to other types of works. (Note that according to the FSF, which is the author of the GPL, you’re actually allowed to modify the text of the GPL and create a derived license if you remove the preamble and you do not call the results General Public License. See the GNU GPL FAQ for more information.
