Okay, so if I understand you correctly, I’m more or less correct concerning competitive play: if it’s not in the event description, the opponent can’t just say “I vote not to use mysterious islands in this game” and roll off? Otherwise it sets a precedent and could potentially lead to “I don’t want to play against forts/events in this game. Let’s roll off”.
It’s not really a situation that should ever come up in competitive play. If something isn’t specifically excluded in the event details, players can’t force a roll-off just because there’s something they don’t want to use.
The main purpose of that was for rules disputes when there isn’t a clear answer, but at this point even that should be so rare as to be almost a non-issue, at least when using the official rules. It’s only mentioned in the PC when dealing with house rules and variants, where sometimes the changes that were made can break unexpected things elsewhere.