It's easy. The same 1% rule applies here, in the sense that most of your customers never have seen the sort of money that laundering would involve. But of course, banks are tripping over themselves to make their short list of richest customers happy, when these are the shortlist that need the most scrutiny. It's a case of perverse incentives.
If you make a deposit of $10,000 in cash that's considered suspicious money laundering. That's one third of the median US salary. I would hardly consider that kind of surveillance to be limited to the 1%.
