Earlier quoted context omitted.
If you ran an establishment where people would come in with big bags of cash with dubious source, swirl them around in a big vat, and someone else would come in with a chit to take out some cash from your vat, I am sure your establishment will get investigated and shut down. In this way it's no different from the real world. Just because it's online doesn't make it ok.
Can you show me any statement in US law where such a practice is illegal? What you're describing sounds exactly like a casino.
Furthermore, actions are not usually the things that are illegal. Usually the law prohibits actions which cause a specific effect. Like murder or pollution. You dont get one free murder every time you figure out a new way to kill people.