Yeah, I was aware of the peculiarities of this particular case (the tribe not wanting contact) and chose to ignore them in order to make the larger point.
Science fiction is great. It lets us look at some of these issues without (perhaps) so much emotional heat.
So let's assume that in the 1950s, aliens contacted the president of the United States -- a theme which recurs over and over again in sci-fi. The president decided that we were too primitive and paranoid to handle contact with a galactic civilization, so he told them to leave us alone.
What I'm saying is that whatever the nature of the leader, whether they are the president, or prime minister, or head of NATO, or head of the UN, or whoever else, it doesn't matter. They don't have the moral authority to be making decisions like that for me. No part of the social contract allows leaders to make decisions of such huge import and keep them secret.
So while I support any individual's right to be left alone and live however they like, I do not support people who are presumed leaders keeping information of that magnitude from others.
Just to speculate a bit, if you're the chief of a tribe, or part of the power structure, it probably sounds pretty good to be left alone. After all, who wants change? However if you're a mother who just watched her 5-year-old kid die from something that was easily prevented? You might have different ideas. Keeping the mother uniformed so that you can continue exercising your power over her is immoral and I do not support it.
As you point out, in the real world, these kinds of things are murky at best, with lots of little conditions and details you want to consider. All I'm saying is that it's not acceptable to simply throw your hands up and give up. From what little I know, I think the idea of repeated gentle contact -- as much as it can be done without annoying them too much -- is probably the wisest course of action. This would allow us to be sure we have made a reasonable and polite effort to inform them of the outside world, as well as trying to catch the newer members of the tribe as they are born and grow up. After all, even if you're left alone, it is reasonable to expect that you still have some kind of relationship to your neighbors.