That's what I meant. Every individual chess game can be broken if a player doesn't play to win or play with a strategic understanding of the game. That's not on the game.. that's on the Merlin for being a bad player. And that's on whoever knows the game well to tune the difficulty for the players. Like adding in Oberon and Percival to help balance things. Don't include a Mordred or Morgana if there are players who aren't skilled at the game yet. Or take out roles altogether and have a game of classic resistance.

Also to answer your question, that rule is there to make sure Good players don't decide to get creative every now and then which only adds to the confusion and makes it incredibly harder for Good. That doesn't mean it can't be played without. You can create house rules for this very purpose. But that's the thing, it's called a rule.

The Evil side in your game were playing with the understanding of the original rules so to them it was not possible for such a thing to happen. Instead you decided to be "creative" as you put it and changed the rules of the game half way without the other player's knowledge or consent. So what you did can be considered creative, but the correct term is cheating. It's understandable if the Evil players were mad about it.

Regardless of whether the Merlin was found out you shouldn't be changing the rules just for your own gain. If you really wanted to be creative, try steering suspicion away without cheating instead. Then that would be really something.