There are several issues at play here. The first is probably the biggest issue I have noted on cruise ships. The overbearing entitled traveler, while we all feel entitled to get what we pay for, they feel the rules don't apply to them and demand, stamp their feet and threaten to complain should their every desire not be allowed. They basically intimidate the crew into giving in. The fact this works means there are not hard and fast policies in place and the crew clearly don't feel confident they will receive the backing of the cruise line should a complain be lodged. So many time I have seen people like this and it really makes my blood boil. As much as I wish to intervene the issue with people like this is they truly believe they are in the right and won't be shamed or told otherwise.
The other issue is security training, while I have no idea what kind of training is provided by the various lines, the big issue is events like this is the don't happen everyday. You can train in defensive tactics till you are blue in the face but the training environment and real life situations bear no resemblance. When the moderate resistance you encounter in a training environment is replaced by a person hellbent on causing as much damage to others, kick, biting, scratching, grabbing hold of certain sensitive parts of the body, training can quickly go out the window as survival mode kicks in. Some will run and hide and some will revert to using what ever they can to get the job done. While the kicking is not a good look, I can understand how it came about. You also have to look at the fact the video clip is a brief segment of the entire incident, there is a lot we didn't get to see which is why its never a good idea to pass judgement as you don't have all the facts.