Marissa needs some more high heels - for trampling obviously - and is there any better way to try out shoes than directly on a slave's body?! She walks her slave on a leash into the store, grabs a few pairs of high heels she likes and then just tramples him - right there in the store - with every pair she might like! He has to endure his mistress' weight and feel the sharp heels of her shoes dig deep into his skin. He's obviously in a lot of pain - and humiliated on top of it! Used as a human carpet in public - where the sales person and every customer can see that he's nothing but a slave!
Mistress Megan is a cruel mistress. She hates losers and when one of the people in her team is deemed a loser, he or she has a problem with her. She either chases the loser away or she punishes him in an effort to get him to stop being a loser. That is what she did to this slave when she trampled him painfully in order to make him stop being a loser.