I feel those who bought st a time when it was known that the match would not take place do have a case because the sellers are claiming at the time they took the monies to provide an experience they knew they could not provide. in that event the T&Cs may not protect them. It could even amount to fraud.
This would be the key issue here! When did the promotor KNOW that the event could not take place and that decision was truly final?
No event is certain to take place - anything can happen, which is why every event takes out contingency insurance - and trust me, it's expensive! Storms, flood, star gets a sore throat, riots, political intervention, fire, bailiffs come to seize the leased private jet or sound and/or lighting equipment, band members arrested, band members too drunk to perform, fights breaking out, you name it and I have had to deal with it.
And nowadays, we have Twitter to make events even riskier - it came as no surprise to me or anyone else in the biz that Roger Waters* managed to act like a narcissistic t**t on Twitter (personal opinion) and get concerts cancelled with zero notice! Nine times out of ten, it is the stars of the show that are the cause of the problem - as was the case with this event!
When things go wrong, the terms of the insurance are that the promotor must make every effort to correct the situation and ensure that the event goes ahead somehow. I suspect that in this case, all kinds of last-minute efforts were made to either get that pea-brained boxer's license reinstated or to find somebody else for Eubanks Jr to batter into premature dementia.
And it is a racing certainty that those efforts went right down to the wire and the plug was only finally pulled with hours to go until the referee blew the whistle and Mr. Benn's head was kicked into play.
In the distant past, contingency insurance companies had to pay out on daft instances cause by stupid behaviour by the stars themselves, but with notorious events around a concert by The ******** B**s, in which the lead guitarist's guitars went to the wrong airport as a result of the guitarist's own drunken/drugged behaviour, causing key staff members to have to walk off the tour.
As he was too far-gone to play anyway and a sound-alike was playing for him off-stage and he refused to 'play' replacement guitars and the rest of the band refused to go on without him (not that they were much better!) the guy from the insurance refused to pay. It came to litigation and the insurance company lost.
Since that and other similar cases costing millions, delinquent behaviour by performers is no longer covered.
*"I-I-I have become uncomfortably dumb!"