The fact he didn't know doesn't mean that it wasn't a mistake. Now that we're clear it's a mistake we can also be clear that this mistake is Garrosh and we all know what he's done.
Thrall made a bad choice in allies. This isn't the first time it's happened. Thrall isn't perfect, nobody in their right mind would demonize him for this, but to do what you're doing and absolving him of guilt is just as bad.