Protectorate War, Warmonger Penalty

[Dark_Jedi06]

So in my most recent game, Germany declares war on my city-state ally that I am suzerain of. I promptly declare war on him using the Protectorate War Casus Belli. I go on a rampage and capture everyone of his cities and eliminate him from the game without a single warmonger penalty. I kept all of his cities, but I could have just have easily razed them all without any penalty either.

Surely this isn't working as intended? Frederick never even captured the city-state that sparked the war.