Out of line
We’ve seen how people react to poor drivers, and particularly, poor parking, and we’re established that it is a sensitive issue. Not being able to find parking is upsetting enough, but to see that a perfectly good parking spot was wasted because people park outside the lines is over the line, and these people need to be punished.
This person is not as extreme as the one we saw earlier, who sip-tied a shopping cart to the car, but he has found a more subtle, yet effective, way of telling this guy off. He left a note suggesting this person was bad at parking, and he should practice staying within the line. It’s not too harsh, but it’s bound to get the message across.