Parts of p7zip are nonfree (to name it the rar unpacker). Debian solved it by splitting the package up and separate the rar-part in an own package. With the new snapshot version of unrar-gpl, we have a fully free software rar-unpacker available in portage (not strictly related, but if we remove rar-support from p7zip-package, might be worth pointing users to that).
Solved by removing RAR from p7zip.