Unfortunately, there is no such thing as "off duty" as far as the NDA is concerned, whether you are in or out of uniform. When a rank or rate is given to you, it is made abundantly clear that you are expected to behave at all times in a manner commensurate with that rank or rate - this is hammered home during leadership training. OK, if they were civilians, then this would not have been remotely interesting. However, they are Service personnel, one of which was an officer, and they are expected (sometimes unfairly) to behave in a certain manner. This is a courts martial based on CCTV evidence - they were fighting in a "military" town, so some evidence-sharing between the local plod and the crushers can be assumed - used to happen in Portland all the time. Sometimes being a Service person on the wrong end of the long arm of the law can pay dividends - but not in places where there is a military presence. If this had been another town, they may simply have been told not to act like prats and sent on their way.
Consequentials - they tend to be a day's pay and time, which ultimately are considered fair in that a civilian employer would also expect you to take leave to attend court. The additional "punishments" are also fairly in line with civilian company redress actions. And yes, we all know that you are punished twice for one offence, but as yet no-one has managed to challenge this successfully in a court. This is not consequentials, obviously, they are not being punished twice - they have been handed to the military for punishment rather than being punished in a civilian court for affray/threatening behaviour/breach/etc.
Behaviour should not be a considered action - if you feel that you are unable to hold your drink, and are liable to "out-pin" if you have too much, then logic dictates that you don't place yourself in that situation. If you place yourself in a situation where you are having to think "if this goes pear-shaped", then arguably you are at the point where you decide that your best plan of action is to remove yourself. That has served people successfully for many years on runs ashore. Notice how the officers disappear first, then the SNCOs, leaving the LHs and the ABs to carry on? It's a judgement call, and sometimes people get it wrong.
The NDA is quite specific - they were fighting, and they cannot choose when and where the NDA applies to them. A sad case of "life in a..." which will probably not affect the LH too much, but will sit in the personal file of the officer for a few years to come....