Now I must ask - in this saying, are we talking about a rule that dictates behavior (e.g. No Smoking) or a standard practice ("as a rule") or a theory based on observation?
To the extent that language is ruled by logic, I surmise that the saying is applied to both cases:
If there were no exceptions, nobody would talk about a rule. You don't say: "the rule is for people not to walk through walls."