The laws around things like sex work or drug consumptions, in most countries, are full of grey areas and willfully-contradictory positions (e.g. one can buy but the other can't sell, etc). So as soon as a transaction is categorised as part of a "problematic" economy, then police can get involved. If your bank lets through "hooker money" unchallenged, the police can eventually accuse the bank of facilitating activities which might be, at some level, criminal.