Mistress Lilli does not have time for condescending people. She prefers to deal with people who know who they are and they respect others. When she realized that she was dealing with a condescending person, she had to put an end to it all and she did so with her poo. She did this by turning him into a human toilet. He had no choice but to do what she wanted.
Princess Nikki is not one to mess with. She is a cruel mistress and she likes to torture losers. She prefers to degrade them with her shit because it is a method many of them have never been punished with. She likes how they are degraded and humiliated at the end of it and how they do what she says for fear of her doing worse than what she had already done.
Princess Ray has a thing for shit and she likes to use it to punish slaves. She does not like any other mode of punishment as the others are not as kinky and naughty as making a slave eat shit. If a slave gets used to eating shit and the punishment stops being effective, she chases him away and gets another slave and starts the process all over again with the new slave.