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.
Pour encourager les autres. If there's illegality going on at a company and the board doesn't know about it, future boards will make sure they are better informed.