What say you?

A pareto-optimal outcome in the prisoner case is one where both prisoners remain silent. Are you saying the external factor or force is the justice system? I'm not sure why it's an issue and I don't think it would have any implication for ethics as a whole-- the external factor is something specific to the prisoner case, you can model other situations which don't involve an external force or restraining authority using game theory conventions.

I'm unsure how well decision predictions using game theory actually hold up empirically but I'm sure it's helped us make some sense of how people make decisions when they are acting in a self-interested and rational way.

Some games require mixed strategies for pareto optimality. If such games exist in whatever context ethics is considered, then ethical theories which only proscribe pure strategies couldn't suffice.

Categorical imperatives ala Kant, for example, seem like the ethical version of a pure strategy. I suspect ethical language is flexible enough though to equivocate between 'pure' and 'mixed' strategies.

Global warming is a nice example of the prisoner's dilemma, where cooperation gives the best outcome, but with a better personal outcome for ratting out your fellow humans.