The counter argument that you are not entertaining is precisely the argument that you should entertain. This behavior is called tit for tat and it is often the optimal strategy in many game theory situations. There are some evidence that people behave according to tit for tat, that is they are more likely to take advantage of an actor that has previously acted in bad faith.

I don’t think this is a demonstration of how frail people’s morality is. But rather yet another demonstration that peoples morality can be explained in big part by game theory.