Mistress Mia had given a sensitive matter to this guy to handle and she was shocked that he was unreliable. She needed someone she could count on and so she punished him for it. He had to taste her shit and swallow it before he realized that she meant business and that she would never let him get away with it. And that is when he changed and became reliable.
Lady Luci wanted to piss off her slave and so she used her shit to do it. The slave deserved it because he was not being a good slave and she felt it was important to show him what would happen to him if he continued behavior that way. He got scared of what she did to him and that is how he made the adjustments that she wanted him to make.