Bitcoins may get a pass because they aren't technically "money", but in general any business that transfers money on behalf of another entity without knowing ex

Bitcoins may get a pass because they aren't technically "money", but in general any business that transfers money on behalf of another entity without knowing exactly who both the sender and recipient are—and registering as a money transmission business, a very expensive process—will be considered to be involved in money laundering. Even if the money is provably "clean" to begin with. A company that implemented anything like a "tumbler" for USD would most certainly run afoul of anti-money-laundering regulations. It's not right, but that's the way the rules are written.