I've seen people make analogies between Lemmy instances and email servers, but I don't think it's a great comparison. I think a better one would be ports on a trade route in the old sailing ship days. People are from different port cities, but they get on a ship and go to other ports and trade/interact with the people there.
So with that analogy, the port city of Beehaw is one with more restrictive laws about behavior: if you aren't generally respectful of others, they toss you out. There was a sudden influx at Beehaw of crowded ships from the ports of Lemmy.World and sh.itjust.works, both ports with less restrictive, more wild west attitudes. They found that their sheriffs were spending all their time tossing people who came from ships with one of those two flags, so they decided not to let those ships dock at their port anymore, or let their ships dock at those ports, for now.