I'm not sure. You do automatically imagine that a player wont purposefully go in with his head just to foul a player. It hurts. So every duel is a fair game where accidents tends to happen and they usually effect both players. Not sure it is an official rule, but common sense really. If a player like Suarez or Ramos did it then maybe the ref would give it, seeing as the classic one where Zidane did it gives a clear red. It is all about intention in this game, and of course narratives. The fecking narratives.