Dude went into an off limits area to make a forbidden contact with a protected tribe. One of the reasons they were protected was to protect them from diseases they had no protection from.
They were known to be hostile - which is why the local fishermen he bribed dropped him off a distance from the island.
He wanted to play by Big Boy rules - and lost.
Another article with more info
https://www.france24.com/en/20181121-us-tourist-killed-arrow-shooting-indian-tribe