Kicking players in the head is definitely not ok for sure, but in reality it was little more than a clip from soneone who was off balance trying to find their feet. I'm aware that the rule book makes no such distinctions between a full kick and my idea of a 'clip' but I find it a bit inconsistent that players fighting and shoving and judo throwing off the ball is disregaded but the moment youy make any direct contact via foot or hand it's an entirely different matter often seeing a card.
Personally I don't like the whole thing of penalties reversed because of discipline etc. I know this case was different because Poite hadn't spotted Itoje's offence, but in general I support the idea of, if deemed necessary, punishing retaliation with a card - but letting the original penalty stand (if there is one).
Otherwise you just regularly end up with situations where the infringing team is not punished at all. In fact that they probably know that they can and will do it again in future providing they are able to make the opponent retaliate and reverse the penalty!