For U.S. lawyers, the reason why “a lot of this stuff is legal” is rooted in respect for the attorney-client privilege, and the Model Rules of Professional Conduct, enacted to some degree in 49 states. In the U.S., “a lawyer shall not counsel a client to engage, or assist a client, in conduct that the lawyer knows is criminal or fraudulent.” Thus, a lawyer here can turn a blind eye, as long as he doesn’t know that the client is engaging in a crime or fraud. To be fair, the American Bar Association has recommended that lawyers not turn a blind eye and do “client due diligence.”

Source: Will Lawyers Face Regulatory Scrutiny after the Panama Papers? (Perspective) | Big Law Business