Because it wasn't his personal life.

Having an affair with a spy/$8b customer who has targeted you and may make the company complicit in committing actual crimes is very much a problem for the board.

Having multiple affairs with employees that open the company up to reputational and legal liability is very much a problem for the board.

In other words, he was doing this while he was making them money in a way that can affect the bottom line, therefore it is - literally - their business.