We should first examine the origins of morality. That will allow us to understand the goals and motivations behind rules. If one of the goals of a moral system is also to get across the message that "We are in charge, remember it and don't mess with us or with the system", then deliberately vague rules are a feature of the system for those that can benefit and wield the law and social opinion against others. Rules can be interpreted to the advantage of the group that runs the show.