If you haven't caught on, this site is owned by Gelbooru as well now. While gelbooru does have the "japanese only" rule, Xbooru is meant to be their anything goes version. So simply put, that is what it is!
There are plans for the future to make it so anytime something is deleted from Gelbooru for not being Japanese, it could be sent here, unless it was deleted for breaking a law, or DNP. We shall see.