Banks are not only obliged not to participate in criminal activity, the way the laws work is that they are increasingly required to do the policing themselves. If a bank does business with some guys and it turns out the guys are money laundering, that can be the bank's fault. For instance, one of the charges against HSBC is that they built up a backlog of fraud alerts. Another charge is that they didn't tune their fraud alert levels in a manner requested by the government. HSBC is also charged with actual money laundering but these two charges I mentioned are not money laundering in the sense understood by most people. And the more ethical reg execs at the more ethical banks have, among their top priorities nowdays, avoiding charges like these. There's even been a sort of reverse regulatory capture: top bankers see among their ethical responsibilities the job of being the world's money police.
This is a natural consequence of voters demanding politicians "hold banks responsible" when their accounts are used for nefarious purposes. Many HN commenters, I've noticed, share the same view. Well, be careful what you wish for. A regulatory environment in which banks are liable if they handle digital cryptomoney from who-knows-where is a regulatory environment in which they will refuse to do so. The knee-jerk urge to paint big banks as shady criminals will only intensify, not mitigate, the regulatory pressure.