Because it's part of an investigation. Again this isn't proof of anything. He may have not ever used this information at work yet and so this is an issue between him and his former employer
Can you share more details about the investigation?
Typically the staff member being investigated would be placed on administrative leave during a lawsuit like this, even if they did nothing wrong. The fact that it didn't happen makes me think the investigation isn't being conducted properly.
They are currently in a suit with Apple over this. If they fire him and Apple loses the suit and a jury decides he didn't even steal documents (a possible outcome) then he can sue for wrongful termination.