Because they should have known. They should have made it their business to know. Looking the other way on purpose should not be a valid legal defense.
> Looking the other way on purpose
Hell even if not on purpose. The point of a board is oversight. If management can commit illegal acts without the board's knowledge, the board has failed. Of course incompetence in a board role shouldn't lead to prison. But it should at least bar someone from future board jobs for a few years.