Mistress Nica had a score to settle with her ex and she knew that there was no easy way to do so as she had tried many times before without much result. So she went out of her way to come up with a new way to do it. That new way involved shitting on the guy and she did so without any care in the world. It worked and the score was settled.
Mistress Nica used her pee and her shit to punish her slave and she made him regret what he had done to her. She was disappointed in him and that is why she made sure that he learned his lesson the hard way by making him a toilet slave. The poor guy had to drink pee and also eat shit. None of those things was negotiable. He had to do them all.
Mistress Nica and her friend wanted to humiliate this guy because he was a nuisance. They went ahead to caution him that they would not tolerate him continuing his nonsense and so they gave him a last warning. He did not adhere to it and had to be punished as a result. That is how he found himself eating their shit and drinking their pee. He never repeated his mistake after that.