I disagree. Retallick is going to try and hit the defender with as much power as possible to get over the gain line. Even if Porter isn't going forward with momentum the fact that he tackles upright still leads to a significant impact, hence Retallick's cheekbone. It's Porter's responsibility to tackle lower and avoid a head impact, especially as we're seeing the results already of players who have taken too many hits (Ryan Jones and many others).In fairness you could see the massive difference in force exerted in the collisions on each occasion
I actually dont mind this, I mean I hate head contact and I like the penalising of it, I think lower tackle heights which will result eventually create a more offloading based game of rugby and we'll see some real high skills, but in the meantime we dont want every touch of the head being red carded, this is a game where blokes are running hard at each other and accidents happen. In looking to absorb the collision you could see that porter didnt impact the head with the same force as tu'ungafasi, who although accidental was completely out of control in terms of his body and its momentum. Interesting though that both happened on inside balls, which a tackler wouldnt necessarily be as aware to and have the time to adjust his tackle height down (because he doesnt naturally expect to make a tackle). Similarly I thought last week when Hansen was in the air and hit, should've been penalty only, no attempt to hit the player, just a charge down with a lot of hang time and speed...
Personally I'd go red for head on head where the tackler is upright as it's not a legal tackle and even if they are absorbing the impact it can still result in serious injury or concussion.