This is not quite correct. Senior bankers (and certainly senior institutional salespeople, of which Greg Smith was one) are typically required by their firms to be licensed by FINRA. Certainly not all investment bankers or traders are required to be licensed (and I don't think they should be), but a significant fraction are.
Also, fwiw, mortgage brokers and lawyers are required to be licensed in almost every state. Licensing--or lack thereof--has little to do with ethical behavior.
Licensing almost always includes a legal commitment to specific ethical behaviors, fiduciary duty and the like. That is one of the main differences between licensed professionals and flea market vendors. Doctors, lawyers, and engineers all make specific ethical commitments in order to win authorization to practice.
That's true, almost every license for any profession (including everything from accountants to sports agents to, in some states, interior decorators and hair braiders) has an ethics component.
My point is that it's trivial to find examples of unethical behavior by folks with licenses, so clearly the license alone isn't sufficient. Many bankers are already licensed, and licensing won't change the behavior of the ones inclined to take huge risks. Maybe it helps at the margin, but it's not a cure all by any means.
Also, fwiw, mortgage brokers and lawyers are required to be licensed in almost every state. Licensing--or lack thereof--has little to do with ethical behavior.