Lady Despina felt that this loser had to be punished and dominated in a way which was worse than he had ever expected. That is why the mistress used her cum eating instructions to dominate as well as degrade him. He had no way to do any of the things she wanted other than to eat his own cum. And the mistress had warned him that he risked being made to eat someone else's cum if he proved difficult.
Lady Despina loves to do crazy things and that is why she used her cum eating instruction to humiliate and torture this loser. The mistress was very interested in all of that as she was bored and humiliating him was to make him learn his lesson as well as to enable her have fun. The guy could not do anything about the humiliation he went through and he had to endure everything.