I was just curious about the "not authorized for use" tracker error message I've just noticed in one of the Ubuntu torrents I'm seeding - and I tend to believe this is the error message trackers broadcast for obsolete or unsupported version torrents. Because at that time I was seeding both
ubuntu-16.04.2-desktop-amd64.iso. Both were using the same tracker and only 16.04.2 had this error message, 16.04.3 was fine.
So it seems this is the way tracker is letting the users know this version is not the most recent one and they should neither download nor seed them anymore and should rather look for current version torrent. It makes sense, although it wouldn't hurt to state this more explicitly somewhere, my two topmost google query hits were for this thread and a similar one on ubuntuforums.org...