They do, however, have the responsibility to set the tone regarding their corporations' risk-taking and to oversee the internal control processes so that they can reasonably expect that their directives will be followed. They also have the responsibility to hire individuals who they believe have integrity and can exercise a high level of judgment and competence.
In the light of recent events, I might add that directors have the further responsibility to periodically determine whether their initial assessment of management's integrity was correct. Indeed, beyond legal requirements, boards of directors and managers of all firms should periodically test where they stand on ethical business practices.