Mistress Bastienne is the kind of girl who does not take chances when she wants something important. She was sure that this guy had top secrets which she wanted to lay her hands on and since she did not want to take chances, she went straight to dominating him to get the info. So she shit on him and as he ate her shit, he gave her access to the info.
Lady Shay felt that her friend needed to be degraded and she did not hesitate to do it. The mistress chose to humiliate him because he had done some crazy shit to her. He had betrayed her and she was so mad that she made him eat her poo as well as drink her pee. And she did it on camera so that she could post it on the internet if he ever pissed her off again.