Agreed, and since people will sometimes let a ref off the hook, if the first one was soft. I've just gone back to look at the challenge which Hwang did get a yellow(24th min) for as well. No such excuse here, it was almost as bad as the one on Walker if anything: Studs up, way above knee level, bringing his foot down on Akanji's foot after he'd volleyed the ball.
The fact that MOTD ignored this is annoying too. If it happened against any of their darling clubs, they are all over the ref about this, with their violins out for the red shirts. It's a disgrace, as Drogba would say.