Lady Tassia wanted to humiliate this guy because she was pissed at him. She felt that he had wasted her time by not satisfying her. She was so angry at him that she felt that the best thing to do to him was to teach him a lesson she was sure he would not forget. That is why the mistress facesat him, trampled him and also used an improvised funnel to pee on him.
Lady Tassia wanted to put this slave in his place as she felt that he did not know his place. So she went out of her way to force him to drink her urine. In addition to that, the mistress also forced him to lick her pussy and he had to do it until she got an orgasm. That was when she let him go. But he had gotten the message.