I already said this on Facebook, but I might as well paraphrase it here. Social feedback loops in American prisons are supposed to be pretty tight: unless they went to some trouble to blind this after the fact (and it doesn't look like they did), it would be obvious whether participants had cooperated or defected. Especially in the non-iterated version (smaller result space), which is the opposite of how it's supposed to work.

On top of that, the social dynamics are a little different from most PD-like problems we encounter in that there's an adversarial relationship baked in: usually you're pitted against someone with whom you have no special kinship, but here we have outsiders setting members of a ready-made insider group against each other. (The original formulation of the problem does have this feature, but it's usually been ignored in analysis.)