So what ends up happening is that the precedents form of a body of rules anyway, for interpreting the rules that are explicitly set out. E.g., "You'll get warned if you're caught saying 'Go ER'" was an interpretation of an existing rule; people had to get warned before they realized that, even though there were a lot of people saying "Go ER," so that it might seem like the mods didn't mind it, that was just because it wasn't always getting reported, and you could get a warning if a mod saw you saying it.
So threads like this become a de facto setting forth of new rules, as people ask for clarifications, lay out their arguments, etc., and mods say how they're going to apply the short set of official rules. Either way, it ends up being a long list, where you either have to read a long list (if they were all laid down as official rules) or you have to read the ban discussion megathread or hang around the site long enough to get warned for overstepping various boundaries, or hear about warnings that people have been given, and thereby discover how the rules are going to get applied in each kind of case.
The U.S. Constitution is a pretty short document, but the case law is voluminous. E.g. the
Fourth Amendment is only sentence, but at a law library, there will be two thick volumes of precedents about how to properly interpret it, that people are expected to know, since ignorance of the law is no excuse. The Fourth Amendment uses words like "unreasonable" and "properly" which is an invitation to use common sense, but in practice, one person's common sense can differ from another's, so what it comes down to is one has to learn what the founders' and judges' (or in our case, admins' and mods') philosophy and past decisions are and take one's cues from that.
Of course, some people have no need to know Fourth Amendment caselaw, because they don't do the kind of stuff that would attract attention from the cops anyway. Similarly, some people just by nature don't tend to say or do stuff that would overstep what mods consider the boundaries of what's acceptable, so they don't really need to know what all the rules and precedents and philosophies are.