Legal doesn't always mean "right". And it was my understanding that the " anglers" didn't keep any. Though they may have signed their consent (via the entry form) to have any fish they brought in kept by the host of the event.
If that is the case then you are correct, nobody has a reason to complain.
Not having a legal reason won't stop ME from complaining though, cuz I still think it was a poor choice made just for the chance to win a few bucks.
Since you brought up the "legality" of things let's consider that IF the fish weren't released unharmed then anyone who culled a fish that day was stepping on the LEGALITY line pretty hard. Read your culling laws.
Can I disregard the laws on culling if I merely give my best 5 fish of the day to my uncle Tom ?