so...do you oppose trying to cure "natural" diseases as well? After all, disease is the "natural way of things."
What makes it different about being a primitive culture, as long as the Federation has the resources and knowledge to save them?
The point is always this: can the culture handle it? In Homeward it is made very clear that as soon as the people found out about the Enterprise rescuing them, it would destroy their culture for good. Same thing happened in Who Watches The Watchers. Same thing in Pen Pals. The civilizations could not handle alien contact. So either rescue them without letting them know that you rescued them, or let nature follow its course.
If you come down to a primitive culture and cure them from a disease, you might end up being their new god. And we all know how that causes death and destruction everywhere. In the long run, you might have saved people from a disease, but destroyed their entire culture with it.
It's a basic moral dilemma. Like "would you kill one person to save a billion"? And well, the general policy simply is: you don't.