If we word the rule like so:
"Slurs directed at people or groups of people are not tolerated.", which is just a more specific wording of, "Insults and hostility to other users will not be tolerated. Insults and hostility against segments of the fanbase will not be tolerated."
The problem is, slurs are intrinsically targeting a group of people when used, the person using them is both implying that the target of the slur (who may be fictional) has something wrong with them, and that everyone labelled by the slur (at least some of who are not fictional) also has something wrong with them. Guilt by association works both ways.
The opposite is some neutral term like "asshole", which isn't ok to use to describe other forumites, but would be fine in fiction as it doesn't disparage a group of people by using a name for them as an insult.
Another example, say there was some user named Benjamin, if people started insulting this user by using their name as an epithet, "Obama really Benjamined that up!", this would be an insult against another user, even if the "target" of the insult is not a user or a fan, or possibly not even a real person.