Why do they get an exception? Serious question -- it seems ethically dubious to me to both refuse to hold them to legal standards, thereby treating them as having the moral awareness of chimpanzees.
If you're asking why they've never been contacted: we know that doing so would be irreversible, and anything we could learn from observing them would be tainted with our own culture as it inevitably seeps through.
Even if this wasn't the case, morality is not universal and we have no basis on which to judge them.
> There’s no point in acting surprised about it. All the planning charts and demolition orders have been on display at your local planning department in Alpha Centauri for 50 of your Earth years, so you’ve had plenty of time to lodge any formal complaint and it’s far too late to start making a fuss about it now. ... What do you mean you’ve never been to Alpha Centauri? Oh, for heaven’s sake, mankind, it’s only four light years away, you know. I’m sorry, but if you can’t be bothered to take an interest in local affairs, that’s your own lookout. Energize the demolition beams.
Is one of your premises that morality is something that has an objective scale and ours is better developed than theirs?
Non-agrarian societies around the world tend to have cultures/religions more similar to each other than to ours, in part because it's more advantageous to how they live their lives.
As an example: a lot of hunter-gatherer cultures have a way of committing what we'd consider infanticide. Agrarian cultures would tend to frown on that since "all humans have a soul", but the reality of moving around with limited food means that you have to find some way of controlling population.
Another example: the way we treat livestock might be appalling to hunter-gatherers since "animals have souls too". But farmers need to raise and slaughter herd animals to survive.
> uncontacted tribes live outside of national and international laws
Also consider that maybe they have their own laws that you and I are in violation of, who's laws should we follow?
Uh, yes. I'm not a moral relativist. If you are, I'm sure you're aware of the tricky outcomes of that approach. For example, if you're a moral relativist, then you can't remain consistent with your beliefs and still criticize US executions (I find that most moral relativists reasonably disagree with the US death penalty).
I'd flip it around and ask how you bootstrap a moral system for all humans (or is it all life?) from scratch. It seems unlikely that I, of all cultures and times, am the pinnacle of morality in the same way that it would be unlikely that I just happened to be born into a family that practices the one correct religion in the world.
Maybe you'd think that I'm inhumane or something, but my beliefs are largely the results of a lot of traveling and meeting the moral absolutists all over the world tell me with the same level of conviction what is right and wrong.
It would be like aliens monitoring Earth and trying inhabitants for violating intergalactic space law while none of the inhabitants are even aware they are part of it.
The question is whose legal standards.
They will have their own law, in their own jurisdiction. Human communities spontaneously develop language, trade and law. You can even verify this in the microcosm of your own communities, with jargon and customs.
And how does their "catastrophic" war compare to the wars of "civilization"?
In terms of deaths per capita, many small hunter-gatherer societies are more violent than our worst wars. It just doesn't look quite so horrifying because the societies themselves are so small, the absolute numbers to us don't even rise to the level of "war". On top of that, the conflict tends to be more of a constant state of affairs that's always in the background, rather than lengthy periods of peace interrupted by an explosion of violence.
Imagine you get into a time machine and go back 10000 years. Do you try them in modern court? I wouldn't think so.
I would try thinking of these people as in a time warp. They are uncontacted, after all—they're not murdering Paulistanos, they're murdering others in their time warp.
Do they not get to have their own territory, or do they have less right to sovereignty than the US or Argentina?
The Brazillian government is probably going to kill a whole bunch of the tribe they are trying to save by making contact with them simply through the diseases they will bring.
As far as I can tell, it's highly speculative third-order conjecture that (1) large numbers of them died due to diseases acquired through contact (2) they have a belief that quarantine will protect them (likely despite not having a germ theory of disease) and (3) this is their reasoning for killing outsiders. Medieval Europe was significantly more advanced, and they had all kinds of strange ideas about the cause of diseases and plagues that prevented them from mounting effective quarantines.
We can't expect them to have the same feelings we do about killing outsiders, and I think it's a stretch to project our understanding and values on them and then to back out a reasonable Western-style justification for their actions. I think it's much better to admit that we know very little about their system of moral reasoning and give them the benefit of the doubt that their killing of outsiders under the specific circumstances they found themselves in is moral within their moral framework.