Mistress Gaia found out that these guys were insincere when dealing with her and she was very pissed. She was so mad at them that she had to punish them. She was not going to let them get away scot-free. That is why she did it and it was cruel and humiliating for the guys but the mistress did not care about any of it. She made sure they never forgot it.
It is lady Scarlet's experience that new slaves needed to be dominated so as to teach them an important lesson. It is always easier to do that and send them a warning as opposed to waiting for them to commit a mistake and then punish them for them to learn. So the mistress used her foot domination fetish to deal with the slave and she got the results she wanted.