Arch also has no policy about not distributing nonfree software through their
normal channels.
Fedora does have a clear policy about what can be included in the
distribution, and it seems to be followed carefully.
https://www.gnu.org/distros/common-distros.html
That makes a difference. Substitute Fedora's kernel with Linux-libre and you
have a GNU/Linux distribution that is as free as Debian GNU/Linux (without
enabling the "contrib" and "nonfree" sections of the package repository). On
the contrary, with Arch, you may end up installing proprietary software from
the default repository.
About third-party package repositories, the distributions are not responsible
for them! Adding to Trisquel's repositories a PPA with proprietary software
is easy. Of course, the Trisquel project does not recommend to add such
repositories (distributions doing so should be criticized for that). See
https://www.gnu.org/distros/free-system-distribution-guidelines.html
