It's dangerous to try to answer "why". I could easily rationalize an answer, but as the article shows, people's reactions are mostly cultural.
Keep in mind that the experiment is not framed as "Person A is giving you $X out of his money" but "Person A chooses the split that you and he get from the researcher's money." If it were marketed as a gift, responses to the experiment would probably be very different (including my own response.)
Keep in mind that the experiment is not framed as "Person A is giving you $X out of his money" but "Person A chooses the split that you and he get from the researcher's money." If it were marketed as a gift, responses to the experiment would probably be very different (including my own response.)