They shared with their kin but rarely traded with outside groups. [...]

[In the Ultimatum game] the Machiguenga rarely refused even the lowest possible amount. “It just seemed ridiculous to the Machiguenga that you would reject an offer of free money,” says Henrich. “They just didn’t understand why anyone would sacrifice money to punish someone who had the good luck of getting to play the other role in the game.”

Um, so, people who rarely engage in trade, are not so keen to establish a reputation as someone who will not be taken advantage of. Even to my capitalist American ears, that seems just plain natural.