This revenge might seem small and petty, but it goes a long way. When someone parks in an unauthorized spot, that might bother the other shoppers, you might get the instinct to scratch their car, or do something worse. However, you never you who you’re dealing with, and this might end badly, and so doing something small, to let them know they are rude, can be sifficient.
In this case, this person surrounded the car with shopping carts, which was a good call, because technically he didn’t even do anything wrong. It would pose an inconvenience for the owner of the car to move them, and it would give him the time to think about his actions.