See you're missing the point.
You think the stick swing happens with or without the punch to the back of the head, whereas it's more than likely related knowing how Geno works. It's never ok to punch any player in the head unprovoked and unprepared. Unless you're living under a rock the last few years, or well ****, a decade + now, you just don't do that and then expect nothing in return, especially an emotional rage type response like swinging a stick or even jumping on the player, etc.
Raffl punched Malkin after cross checking him and trying to rough him up to throw him off, Geno only swings after the punch in the head.
If you are DoPs and you ask him why he did that, you bet your ass he's going to say, ok well...should I just stand there and allow someone to punch me in the head and pretend that doesn't make me react in the most angry way possible?
Not sure if you noticed, but head injuries are a thing and unless you're in the NHL's team of lawyers that thinks concussions aren't serious, a punch to the head like that, especially the back of the head, is never "a part of the game" and especially more so now.