But the problem has to explicitly exclude altruistic behavior; since if both subjects view the consequences to the other subject as being roughly as serious as

But the problem has to explicitly exclude altruistic behavior; since if both subjects view the consequences to the other subject as being roughly as serious as consequences to themselves, there's no dilemma at all. They both shut up and help each other. The whole point is to show a situation exists in which two purely rational, purely selfish actors get an outcome that's actually worse for them from a selfish point of view than the outcome that two altruistic or irrational players would get. That they are criminals "establishes", to use the story-telling term, that these subjects are (unusually) not altruistic at all - the punchline is that being self-serving turns out not so well for them.