Where the league has to be painstakingly careful is doling out punishment in the rare instances where a defender isn't headhunting, but when the receiver (or Quarter Back) hunkers down bracing for the looming impact, which completely changes the attack trajectory dynamics of the defender considerably. Once a defender is locked-on and committed to the play, he can't change the direction of his force, he's already committed.
That makes things incredibly difficult to determine intent... and if the league is determined to remove head shots - then they're gonna get it wrong sometimes. And it goes without saying that the physical dynamics involved (as in the build and overall height of the player being tackled) is sometimes gonna make "intent" very hard to prove. Most of the time it is fairly obvious that a defender is targeting above the shoulders, but not always.
In some cases it is indeed situational. Cam is a 6-foot, 5-inch quarterback, which means his pre-impact disposition might influence precisely where the defender makes initial contact. I don't think that applies to the hit he took near the end of the game when the offsetting penalty occurred, but it just underscores another dynamic that makes the referee's decision a little more complicated. There are indeed other variables that come into play.