If it cannot be proven that he knew about it or encouraged this behaviour then he cannot be held legally accountable. It's like demanding the US president should go to prison because some CIA team somewhere does some really shady stuff that he doesn't know about.
Shareholders might still choose to fire that guy, but it's a choice they have, they do not have to if they are satisfied with his performance.
How come that people today lose their ability to reason as soon as bankers are involved? It's similar to the National Socialists reasoning when they blamed the Jews for everything bad "because they had the all money and controlled the world"