There's a common urge felt by people who make "rules" to enumerate even the common sense ones that absolutely don't need to be stated. Governments make laws against murder, forums make rules against "threatening physical violence", et cetera. This isn't just unnecessary redundancy. There's a deeply perverse idea being subliminally reinforced here.
That idea is that a rule doesn't exist until it's declared by an authority; the rejection of conscience as the source of morality or what some philosophers call "natural law". You don't need to have declared a rule against murder to be justified in punishing it. And once you accept that, you realize that almost the entire concept of formalizing rules is wrong-headed. If the rules are actually justified as a matter of being morally righteous and not a human being's arbitrary decree then they don't need to be specified in order to be morally righteous.
I'm not saying we shouldn't try to enumerate our moral principles - that'd make me a tremendous hypocrite - but that it should only be done for philosophy, not for the idea that they need to be declared for us to expect others to follow them. Note that most of the time when other people declare unnecessary rules they're not actually formalizing them. "Murder merits a lifetime in jail" raises so many questions: what's the definition of murder? Is it equally bad if the person was going to die soon anyway? How does degree of temptation, the victim's merit, and the case of a penitent criminal factor in? My philosophies aim to illuminate universal principles that answer those questions emergently.
Sometimes arguments are made like, "well people still act mean on our forums, so obviously the rule is necessary". The fallacy there is ignoring that "necessary" is a two-part word. Just because a problem can't be solved by other methods doesn't make the countermeasure necessary; a countermeasure is only "necessary" if both the problem is unsolvable by other means and the proposed countermeasure will actually help. And the only people who need to be told not to be assholes are the people who are going to be assholes anyway. (I'm getting reminded of something...)
Another aspect especially related to forums is that if we declare formalized rules, we give the impression that those rules are all there is. There'd be an outrage if someone was banned from a forum for something that wasn't against the rules, but if the forum is owned by an individual (like this website is by me) and that owner doesn't lay out explicit rules, then of course they have the right to control what gets posted on their site and few people would argue with that. The problem only arises when you try to detail a contract which implies that if the other party doesn't violate the contract, you won't terminate your service to them.