"decide over another group which is a subset of the former" A subset of the former in the name of the frontiers set by a centralist monarch who won a war and forbid language and institutions in Catalonia in 1714. Check the law that supports my comment: https://en.wikipedia.org/wiki/Nueva_Planta_decrees

"Otherwise, any city of any country, or any group of individuals, could decide to vote and set their own rules for whatever they want." --> And do you think this is wrong? I think self-governance is the future. Not to go against anyone, but to decide more about everything. Not to put a paper in a ballot every 4 years.