What about this change to copyright.txt? I think even those wanting to misinterpret things should have a hard time doing so and it mentions explicitly that pak sets may have their own licence:
--- a/simutrans/copyright.txt
+++ b/simutrans/copyright.txt
@@ -18,5 +18,9 @@ comments please send to:
team@64.simutrans.com
The source code from the program is available under the Artistic Licence.
+This is a written permission as demanded for above for every right granted
+by the Artistic License. Note, however, that individual pak sets may come
+with their own license.
+
Download the source from https://sourceforge.net/projects/simutrans or
find out about our svn in the forum.
Here is also a patch for the other change to licence.txt I suggested (mentioning the new files in the skins.src/ directory):
--- a/simutrans/licence.txt
+++ b/simutrans/licence.txt
@@ -7,6 +7,7 @@ Simutrans source code is provided by the current simutrans team
This includes the translations from:
http://translator.simutrans.com/
+the images and .dat files included in the source distribution,
the midi files and the fonts with the execption of the m+10r.bdf, which
is taken from the M+ font project:
http://mplus-fonts.sourceforge.jp/
Edit:
And because somebody just pointed to an old thread: Is the `no-sell' restriction intended to be stricter than the one in the Artistic License? That is, is selling intended to be forbidden even as part of a software compilation (such as, for example, a Linux distribution as Debian) as well or just selling Simutrans on its own (as the Artistic License states)? From prissi's comment above I take that it is not intended to be stricter than the Artistic License. I think this does not need an extra comment when the change I suggested above is done.
*sighs* I really don't like discussing license problems :-/