I dunno if this is going to start opening a different can of worms. The NFL has banned helmet to helmet hits.
With Shaw, I don't think he led with his helmet and intentionally tried to make contact. I think it was just a check gone bad, but I think it does warrant a penalty, and maybe a firm warning. It's just recklessness with your equipment. To me, no different from the stick, skate, hard elbow pad, etc., especially if it comes in contact with the head.
Brown however, I think was an extremely dangerous play.
As for Zidane, I think the Italian player was talking smack about his mother or sister (from what I remember and depending on which source), but the big guy sure went down like a ton of bricks. Plus if you look earlier on the play the Italian player pinches Zidane's nipple. Dirty stuff that goes on in soccer. That's what started the "talk" that led to the head butt.
Soccer needs a diving/embellishment penalty.
https://www.youtube.com/watch?v=TaCcB30Zc2Q
Freaking medical staff had to come out too??