Is the argument that even if you'd do such an experiment with consent of the participants, it'd be unethical? Or are we just assuming this will be done without the consent of the participants?
I think the implication is that it would never get by the IRB.
It doesn't matter if it's actually unethical. It just has to be approved by a board.
Or what? The board is going to kick their asses?
Yes. All the time the rest of the organisation spends on mandatory ethical training, the ethics board spends on martial arts.
It's a common misconception: the ethics board isn't the committee deciding whether the trial is ethical, it's the instrument they use to punish you if they decided it wasn't and you decided to ignore them.