Isolated tribes in South America should be allowed to continue living undisturbed for two reasons. First, having not been exposed to the rest of the world, their immune systems may not be able to handle the exposure to various new pathogens, viruses and bacteria. Second, these tribes have consciously chosen to remain isolated, and their choice should be respected.
Uncontacted, indigenous tribes should definitely be left alone. Their way of life would not survive contact with our culture and there is no benefit to diluting the already homogenous cultures of this planet by forcing an introduction upon them. They are a precious resource if left alone and uncontacted and should be left that way.
If these isolated tribes don't wish to be contacted with outsiders, it would be a gross violation to force them to do so. They have every right to live their lives on their terms, under the protection of the Brazilian government, however much anthropologists would like to study them. If someone wants to force himself or herself into someone else's home under the excuse of "studying" them, that homeowner has every right under the law to refuse them entrance. Tribes, however isolated, should be granted the same protection.
Brazil is right to want to leave the country's isolated tribes alone. These tribes have lived on their own in isolation for millennia. As long as these isolated groups are not harming others then there should be no reason to force contact with them. Both Brazil and these tribes will be better off if they can leave each other alone.