That's because of context. I think it's better to think of "il faut" as "it is necessary" and think of "il" as the "it" part similar to the "il" in "il neige" or "il pleut". Except in English, "it is necessary" and "one must" are synonymous and often "one must" flows better so is used in a translation.
But the fact remains that if you're given a sentence "One must..." and are asking to translate it to French, you're probably going to begin with "Il faut...," meaning there are exceptions to the rule as you put it, in practice. However, I understand your intention in providing it.