> Why can't Icahn be held responsible for his actions too??
Well, you seem not to understand much about capitalism.
The board can be held responsible because it does not own Yahoo, merely manage it for its legal owner.
Icahn cannot be held responsible for his action because HE IS the owner (well, partly) so he can do whatever he wants with his shares.
In the same way that if you destroy your house (no insurance claiming) because you don't like it anymore, you can't be held responsible, the very sentence is meaningless in that case. But if you appoint someone to manage your estate and that person destroys it, then they will of course be held responsible for it.
And this is regardless of whether Icahn is actually morally justified in doing that, or whether it is actually the right thing even for him: it does! not! matter!