Countries are really big. There are multiple layers of sub-community, providing for much more diversity even in a country that isn't about diversity.

Yes, but you still need standards for said sub-communities to be able to coexist.

It was not done proactively for a broad category of people who had not done anything wrong.

Depends on the website and the situation. Hacker News, for example, temporarily disables creating new accounts whenever it is linked to by a mainstream source. Also if a bunch of people from 4chan decided to show up here, I suspect you'd support proactive measures.

Speaking of those people not doing anything wrong, "don't, won't, or can't accept [the community's norms]" might be a legitimate reason, but it was not the criterion applied in the example, even approximately.

What example were you thinking of? In the example of immigration to the GB, if you listen to the complaints of the people against immigration, many of them amount to the above criterion.