Mistress Anna did not want a dirty slave in her house. So she came up with a way to punish him and he had no choice but to do what they had agreed on. He was forced to become clean and he had to follow her instructions on how to go about it all. He did not want to eat poo again and that is why he had to follow her instructions.
Mistress Anna had to send a message to this guy and she chose to use poop to do it. The mistress felt that anything short of that would not send the right message that she wanted to communicate. And that is why she had to make sure she turned the guy into a human toilet and had him eat her shit as well as drink her urine. The message was home.