Why?
Horrible past experience.
What
Warrior Cats is just a series of books written by someone. In regards to the guideline it is the same thing as saying 'No Star Wars', just for Warrior Cats.
Offended(¿)
You shouldn't be, at least I think you shouldn't be. If you really like WC (Warrior Cats) and want to write about it, do it elsewhere and you'll be fine.
Story
Ah, the Great Purge, and the build up to it... sure am glad I was not around for such harrowing times... Anyway, the story (to try and summarise) was that the fandom (or the parts that were attracted to this site) ended up being the very antitheses of what you would want on a site like this. Many did not care for improving their writing, pretty much rushing rubbish out and cluttering the site. Hostility was also present and directed towards even those who wanted to help them improve. Worth mentioning that many of the stories were pretty much one liners, with fake choices, and copy pastes of the other stories with the names swapped out, thus lack of (or no) creativity. Ultimately, they had to go, so Warrior Cats was banned and it was eventually removed.
From what I recall, there were some among the fandom who weren't that bad, even some who did want to improve, but the environment that ended up being created was simply a bad one. It is a shame that a fandom had to be banned, but I can see why it had to be done, and hopefully, so can you, now that you have read this second hand simplification that probably missed some important details.