I'm interested in the inconsistency of those who accept defection as the rational equilibrium in the one-shot PD, but find excuses to reject it in the finitely iterated known-horizon PD.

I don't see the inconsistency.

Defect is rational in the one-shot game provided my choice gives me no information about the other player's choice.

In contrast, the backwards induction result also relies on common knowledge of rationality (which, incidentally, seems oddly circular: if I cooperate in the first round, then I demonstrate that I'm not "rational" in the traditional sense; knowing that the other player now knows this, defect is now no longer the uniquely "rational" strategy, which means that maybe I'm "rational" after all...)

Maybe rejecting common knowledge of rationality is an "excuse" (personally, I think it's reasonable) but I don't see how it's supposed to be inconsistent. What am I missing?