I'm actually working on a research paper at the moment which shows how this case is overstated. The IPD merely shows that given a situation in which neither party has any advantage over the other the best choice for maximizing outcomes is cooperation. In this sense the prisoner's dilemma "begs the question" or assumes the answer it then provides. There is no "moral symmetry" in this game, merely a set of preconditions (poorly representative of the "real world") which make this cooperative outcome inevitable.