MuhVerisimilitude
Hero
We already know that people don't act rationally yet you assume that everyone does. That's what the will save is for. If they fail the save they stop acting rationally. It's exactly like a fear effect. If they fail the save, they stop acting rationally.Not under any and all circumstances, no. There is no nonmagical way to do that in my view.
But anyway, you are arguing that the targets of this ability are aware that acting on the taunt is against self preservation. That seems like a bad assumption since again it assumes they are rational. What if the point of the taunt is that it makes the taunting barbarian seem like an open target?
You are saying: Not attacking the barbarian is the rational choice, but how do you (or they) know that is the case? There is no reason to assume that, since the characters in the setting are aware of the counter attack ability.