The problem, Gash, is moderators don't know it when they see it. They disagree about what they're looking at.
So let's address that shall we?
Why is it a problem for individual moderators to disagree about where a thread should be? Certainly if we had a codified structure that mods were expected to memorise it would maybe be an issue if people hadn't done so. But we never imposed such a duty upon mods. Choosing where threads should be is a judgement call, and one poorly catered to by a rigid system that brooks no disagreement.
Let's go back to the thread from CAASS that discussed a young female tennis player who was told she was too fat to play, despite being demonstrably good at tennis and capable of competing against other players. This is a thread about sports, so by Avi's continued assertion, this should go in general. But it's a thread about fat shaming, health, politics and gender, and so also fits into CAASS just fine. Where this thread is placed is not a clear cut matter of assigning categories, but a judgement call of to what degree a thread belongs in a particular forum. A rigid definitive structure doesn't serve that well.
Moderators differ in judgement on what constitutes being in X or Y forum, and there's nothing wrong with that. As previously stated, a thread might potentially be appropriate for several forums and may not fall simply into one category. Indeed, there's little point even discussing something that does fall neatly into one category. So mods inevitably end up dealing with the borderline cases, where there is some debate to be had. As such, mods often differ on where to place a thread and discussion is the way we assess that.
As such, I'm not sure why it would be a problem for moderators to disagree or not be immediately certain where a thread might be placed, and I don't see how a written definition of threads would make that job easier or lessen confusion or cause less discussion.