Because he doesn't have to prove that he didn't break any laws; TSA has to prove that he broke the law.

Because it's rude to ask someone where they got their money, and why they're carrying it ... especially, as has been noted frequently, the presence of any quantity of cash has nothing to do with the safety of airline travel.

Because, as James Duane has effectively demonstrated, answering questions from those acting under the color of authority rarely leads to good results. If The Powers That Be don't get you for the original crime, they can get you for something else you reveal in your answers.

We can't know the answer to that question. First of all, I suspect very few travelers have that much cash on them, relatively speaking. ($4700? Heck, I've never held that much cash in my hands at one time.) Second, of those very few travelers, I suspect very few TSOs would've identified that cash as potential contraband and escalated the incident as this TSO did. At that point, we're dealing with a small enough set of people to make any statistics meaningless.