The point isn't about a change of legality. It's about a change of enforceability! In the normal regulated financial system _every_ entity you can interact with is known to someone who is a) good at keeping paper traces and b) extremely motivated to provide them, given appropriate motivation, i.e. a properly formed request by authorised parties. And that limits the number of times you can do anything scammy to "few" before you need to switch identities

So in your scenario: The 3 billion will reappear at some point and they will raise flags. And at that point there is a convenient rendezvous point between law enforcement and the perpetrators. And we can all quibble about whether the authorisation is appropriate or not (not all "requests" are requests and most won't even need anything close to a court order). But it's damn convenient to follow the money