certainly no rule should be completely black & white
But if it's all about "them" (Starfleet) rather than the cultures, then nuances with the situation of the cultures shouldn't affect the situation with Starfleet. If the PD is there to uphold the principle that Starfleet cannot take law and fate in its own hands, then it shouldn't matter whether the lives of ten people or ten billion are at stake. After all, that very type of judgement would be the thing taken out of the hands of our heroes.
Interestingly, Kirk was quite altruistic about saving civilizations from natural disasters without orders from on high: he tries to stop an asteroid from smiting an obviously transplanted culture in "Paradise Syndrome" while apparently very far away from home, despite being aware that getting smashed by big rocks is the natural destiny of this culture, and one deflected rock won't really change a thing. So one would expect not just some sort of standing orders to help cultures in need, but indeed an obligation to re-dictate destiny for hopeless cases, against all reason, regardless of whether they wanted it or not.