Mistress Zhan needed to stop the lies this guy had. He was used to lying all the time and the mistress was not going to let him continue down that path. So she made sure that he ate her poop for him to know that it was not ok with her. The mistress loved seeing how degraded and humiliated he was. He never messed up again after what she did to him.
Goddess Wu was shocked to see the perverted things this guy did in public and since she was the one who had seen what he had done, she had to come up with a way to deal with him. She did not want to pass the buck and that is why she made him eat her shit so that the extremity of her punishment would make him never do it again.
When goddess Gina and mistress Yan tried to punish this guy, he resisted what they wanted to do to him. They were shocked that he had the audacity to refuse and to resist what she did to him and the two mistresses ganged up on him and they made sure he ate their shit, which was not even part of the plan had he played nicer and not pissed them.