Lady Nora was out to teach this guy how to lick spit and she felt that the best thing he could do was to follow her instructions. She had to make sure he became good at it and that he did it soon. That is why she did what she did to him and why she had fun at his expense. He was scared shitless and he did what she wanted.
Goddess Kira wanted to pass a message that she was sure would not be forgotten in a hurry. That is why she chose to use her spit fetish to do so. She was not out to make the guy feel pain. All she wanted was for him to feel humiliated. And she did so after she had teased him with her hot ass and then spat on him to humiliate him.
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.