In the episode The Witnesd, JJ mentions to Michael that he will buy art supplies or something like that when he gets his pay for being a key witness in a car accident. Michael says, "JJ, you most likely will not be getting paid for your testimony.

JJ asks why not and what's wrong with getting paid for testimony?(I was also wondering what was wrong with getting paid for testimony) Michael replied "If someone is going to pay you for testimony, they can get you to say anything they want you to say. It's unethical and immoral." I never even thought about it like that until Michael pointed out the flaw in a to be paid testimony.