that's a somewhat false premise. Those people would be people who bought all of cursed city, not to play the game but to use the enemies part of the game for an army whose battletome wasn't out yet, and who probably also knew about this rule because it was revealed pretty early after release. Also, that logic would, to some extent, apply to anyone who ever purchased models/an army based on the rules at the time of purchase = that logic would mean that changing the rules was always unfair.