Well, I don't understand why you reversed it :-) so not sure exactly what you need explained.
Not knowing what exactly you need explained, I can only say: The documentation sources are part of the code base's source, so building the documentation needs those sources; so the sources need to be in the source distribution.
@BigNose The .rst files were in the .tar.gz, but were ignored by setup.py. They weren't copied anywhere during installation. My (perhaps limited) understanding of the .tar.gz file is that it is only used for running setup.py, in which case those files were useless.
Are there people who do other things with a .tar.gz file? It was never intended to be a complete copy of the working tree.