This also just is the time defederation happens most. When populations grow faster than people can manage.
Taking on the responsibility of hosting a community website means doing what you think is best for they community. For a place with clear rules and established norms, that means upholding those rules. And if you can't uphold them against the sheer number of people flooding in, then it means reducing the number of people.
No one website is responsible to the network. This is not a power trip. Though this is about people protecting their "precious communities", as you so judgementally put it. Because they set up their site to create a coherent community.
If you way to be a part of it, you can apply to join. If you don't, then you're not entitled to interact with them.