About 800-something years ago ason Kyomori Taira noted that it is gratitude that distinguishes the man from the beast. (See "Taira monogatari" book for the quote). These tribesmen do not pass the test.
Maybe they are scared of the giant flying metal bird? They may just be reacting to xeno-phobic tendancies. Like what you see when you say "burrito" on many threads on this site.