because they regard to the tribesmen as animals to conserve. not as humans to interact with like you do with humans, to educate, to help.
now, why don't we regard north korea as the same? I'm willing to bet the justice system in those tribes works just as fine as it does in NK.... the mortality rate goes up? well, how about vaccinating them - though a lot of the reason why things hit the fan might be just that their tribal system collapses as the village elders/strongest no longer have anything to base their power on since they don't have the most strength(since guns take out brute force from killing prowess) and its obvious pretty soon that they don't have the most knowledge either.
(furthermore, the population levels of the tribes are so small that they're doomed in the long run anyways)