The word "can't" means that something is impossible. It is not impossible to enter an undubbed bird in a show. A correct statement would be that it would be DQed.
Like I said, I'm picky about semantics--it is the backbone of communication. If I don't say something accurately, then I shouldn't expect that it will be understood--I can hope, but not expect.
I suppose a reason someone might choose to enter an undubbed gamecock would be to exhibit it in its natural state or to make a statement. Or maybe they don't know the rule.
There was no specific mention of OEGB Shows; I wasn't even aware that there were ones--all the poultry shows I've been to have had many breeds of chickens and often other poultry, waterfowl and gamebirds. Some have pigeons.