Ya. It turns out that even in these games where the person running the experiment tells the participants that there will only be one iteration it doesn't change the results any.
That being said I pretty much agree with you. An experiment with a single iteration is very unlike the real world so it's no surprise that participants pretty much ignore that directive.
Even in the case that the game is a single iteration, it doesn't erase the background fact that society still exists and that punishing people for breaking societal norms can enforce societal norms of fairness.
The experiment doesn't control for the fact that even though it's an isolated game, it's still happening in the context of these people being members of society and hence trying to enforce societal norms even in the game. Society is a large pool of random encounters over and over, so the players are making the correct play given that there's no control of that factor, ie, it's not truly a stand alone game.
I find a lot of the analysis of these games from psychologists leave out details like that which make the responses move from irrational to rational.
That being said I pretty much agree with you. An experiment with a single iteration is very unlike the real world so it's no surprise that participants pretty much ignore that directive.