Don't know your age, but for the past few years -- especially since concussion became a more sincere concern -- they've been cracking down on attacks to the head pretty significantly. Jannie du Plessis was carded yesterday for it, Tomas Lavanini was penalized for a shoulder-to-head tackle against Tonga, and Peter O'Mahoney was carded for a shoulder to the head/neck against Italy.
And that's been a known area of attention not just for months, but for years. I'm sure we've all seen refs penalize players for not using their arms when cleaning out, and scolding the offender for it as well. So unless there's a super-secret part of the game where shoulder charges above the opponent's shoulders are allowed, and some of us just don't know about it, I'm not sure why Hooper's flying shoulder to the head is any different. Had Hooper tried to use his arms and didn't level and aim with just his shoulder, it'd be a different story.
All I'm saying is it was a stupid thing to do, and it particularly stands out because there were at least three other penalties/cards for the same sort of thing in a 24-hour period. Had he at least got his arms up, it could have been a brilliant cleanout.