In “The Wife of Bath’s Tale", there are several reasons why King Arthur might have allowed the queen to punish the knight.  One reason is that the knight raped a young woman, and therefore, the women were very upset.  Allowing a woman to decide his punishment might be an effective way of calming the female citizens of his kingdom.

Another reason he might have allowed her to decide the punishment is that perhaps a woman’s punishment might be more humiliating for him, and he certainly deserves it.  The knight goes off on his quest to find what women want, and he cannot even find the answer.  When he does find the answer, that all women want dominion over their husbands, he then has to stand up and report that to the entire court.

Of course, if you asked the Wife of Bath why the queen was allowed to punish the knight, she would have a different opinion altogether.  King Arthur was an ideal king; it is a given that he always did the right thing.  As the theme of the story is that women should have power over their men, it is likely that King Arthur allowed Guinevere the power to show that he understood women should have power.