Any definition of rationality that only takes into account monetary incentives is completely flawed. Money is a means to some further ends, something that will satisfy some other human desire. If you're poor and starving, I'd imagine you would take any amount because you value your next meal over any chance to exact vengeance. On the other hand, if you have everything in the world, what's $10 over the chance to share your views on cooperation/vengeance with someone that just seemingly slighted you? The psychological satisfaction that results would be worth more than another $10 in your pocket.