I found it sad because their previous life/existence was fascinating; it seemed to be an example of a successful (as measured by happiness) human society that was amaterialistic.
FIY, mandatory schooling doesn't apply to indigenous people. But they're still Brazilian citizens, and our government is obligated by law to offer education to all citizens. Indigenous people (including the Piraha) may also be entitled to e.g. retirement benefits, so it makes sense to learn arithmetic.
I also somewhat fear, from what was shown in the documentary, for their future as in many cases, suddenly introducing a more modern/western way of life to an isolated people often seems to result in social breakdown.
I believe it's best to err on the side of caution in cases like this when you are interacting with a complex delicate system.
While I would consider myself a western liberal, fully appreciative of the benefits of a western scientific education and the use of modern technology to improve human existence, I would still expect great sensitivity when introducing such benefits to a people like this.
I'm not a relativist generally but in this case, the absolutist belief that these peoples' lives will be improved by exposing them to a modern education and technology reminds me that missionaries also had an absolute belief that introducing such people to the Bible (for example) would "improve" them and we know that the results were mixed in the latter case.
I haven't seen the documentary, did anyone say that? My point is if you get monetary benefits, then you need to be able to read, write, and do arithmetic. This isn't a controversial idea. We aren't talking about uncontacted tribes. And it's voluntary. Some people want to get education, and they should be able to. The idea the that the government should somehow deny them that is untenable.
And if you think the government should not offer them social security at all then you have a bigger problem.