Lemmy.world and sh.itjust.works tried to grow their userbases as fast as possible, so they let free account creation without any screening and minimal moderation. That attracted a lot of people, but also a lot of bad actors, who then went on to use federation to cause trouble on other instances, particularly the ones they felt federation allowed them to “sneak into”, bypassing whatever rules.

Beehaw’s “rules” are more of an ideology than a strict set of rules, which needs more work from mods to apply. At the time it didn’t have (likely still doesn’t have) the mod staff needed to deal with that onslaught, so the only mod tool left, was to cut off those instances entirely. At least until they are willing to, and capable of, putting some order among their users.

Lemmy doesn’t even have a way of reporting users to their source instance (a feature that would pose its own set of challenges), so they can pass as model citizens on their home instance, while wreaking havok everywhere else.