I do not work at an investment bank and probably never will, but I can still imagine numerous ways to bury the details behind such operations as to make them appear legal, when in fact they might not be.
What about brokerage houses with many unspecified internal trading accounts? Hook a few of theese to anonymous offshore owners. Then by law there would be no shared profits. If a single trader then is hired to manage these accunts discretionary, he can as he pleases switch funds back and forth. How to define and prosecute painting the tape in that circumstance?
In the world today with all the kinds of creative otc derivative-contracts it is not difficult to transfer formal obligations while retaining actual control.
How could possibly authorities keep track of nested deals and exotics? How many Quants are working for the sec versus investment banks?