It became clear they were exempt after the fact, when nobody was indicted. At the time they had no reason to believe they were exempt, and they still did it.
You're seriously arguing that in our nation's top financial institutions, the only thing that should be keeping them from committing fraud is fear of prosecution? Really? That's how we run our financial industry? That's where you're setting the bar?
Thus, their responsibility ends only where it's officially illegal, no matter how fraudulent? What a great argument for capitalism: Fraud, Unless You Stop Us First.