The post-play headbutt was more unsportsmanlike, but the tackle was the more dangerous play.

The rules appropriately eject a player who tackles with their crown of the helmet in order to try to actively change how players play the game. That play, while not necessarily a dirty play, is dangerous for both the tackler and the ball carrier. So they make the player really pay the price by missing playing time, even if the player was not doing anything intentionally wrong.

Their running back's headbutt was unsportsmanlike and silly, but wasn't really a dangerous play. I think it was appropriate to give him an unsportsmanlike conduct penalty. Remember that if he did that same thing a second time in the game, he would be ejected.