Mistress Nyx needed to teach this guy to do things her way and that is why she chose to teach the guy a painful lesson. She had to ensure that he would never forget it and that is why she put him in his place by spitting on him after she had tied him up. She had a whip on her hand ready to strike if he did not toe the line.
Goddess Diana needed to degrade these guys and she did it in a way that they had never had it done to them before. She used her spit to do it and she humiliated them a great deal. They cried and begged her to stop especially when she added her slaps to the mix. The mistress slapped them in between her spitting and it was painful and humiliating for them.
Mistress Jennyfer noticed that her slave was kind of disobedient and she did not like it. She had to make sure that habit ended that very day. So she used her spit to do it. He was forced to open his mouth and the mistress spat into it. The mistress also lay on him and let him admire her big tits but then denied him and left him high and dry.