The thing is, when these officers are training, they're know they are 'training'. They know at that time, that barring an accident, no-one is really going to shoot or hurt them intentionally.
How often do these officers get caught up in a real-life violent kick-off situation, where there is a crowd of people, in an airport, and everything is unfolding uncontrollably in a few seconds?
The whole thing is over in an instant. No time to politely ask the suspects to submit to arrest if they wouldn't mind. No time to reason with violent thugs who are clearly intent on bashing up these police officers. No time to allow an incident to escalate that could result in numerous people being shot or otherwise injured.
Training can only go so far. What we don't see is the probably dozens of routine incidents akin to this that the police must handle successfully day in, day out.
It's a shame that many police forces have a reputation for being lenient on their own officers. I don't envy anyone dealing with this awful situation however.