Bill Murray gets the big picture: life's too short. He simply doesn't give a crap either way. Suing people over a tee shirt is a waste of time.
If this were the same for some other person who sees that money is being possibly taken out of his pocket because of someone else using his likeness without his permission, and he isn't quite set for life money wise the reaction might be understandably quite different.
His lack of an agent and the difficulty of getting him to agree to actually do a film suggests that money is not a motivator for him generally.