All of these services are very new. Exactly what people want out of them — including what the people operating instances want out of them — is still being worked out.
This is not a commercial production service that you have a contract with. It's an experimental system run by volunteers who don't all have the same ideas in mind. People aren't just working out the kinks — the process of discovering what this is all really for is still ongoing.
To clarify a minor point, beehaw isn't new. It was established in Feb 2022, and it's been thriving with a relatively small community up until this months crazy growth. They're not so much finding their feet as trying to maintain an existing communities safety in the face of rapid growth.
Beehaw is meant to be a safe space, mitigating toxicity, while other instances, by having registration open to all without moderation, causes that.
By being federated, they can interact with one another. Beehaw defederated them in order to avoid that. The main argument being that the modding tools right now aren't good enough to help them do it any other way.
The short version is that beehaw was struggling with the (currently) limited toolset available to moderate user content, and they saw a heap of users posting things they don't allow on their instance were coming from the two other big instances, so it was more effective for them to defederate to try and stem the tide.
I imagine regeneration will occur in future when the lemmyverse stabilises a little, and when better mod tools are available
beehaw.org blocked lemmy.world. which works like you'd expect blocking someone would work. they can't interact with each other, can't participate in each other's communities, etc.
lemmy.world still "has" content from beehaw before the block happened. you can still see it and interact with it, but anything you do on beehaw communities that you have copies of won't get synced to beehaw, and thus can't be seen by anyone else on the fediverse.
you shouldn't be getting new posts from beehaw at all, other than comments inside of non-beehaw communities (such as in a kbin magazine where a beehaw user comments).
lemmy.world has had a handful of back to back queerphobic trolls spamming hate across multiple groups and instances.
They would get banned and come right back.
The reason they were able to do that is because of the open signups on lemmy.world.
Beehaw is an instance that takes protecting their members as their highest goal. They value it significantly more than wide federation.
And so they blocked lemmy.world, as it was a source of bigotry towards their members, and there were no other moderation tools available to them to resolve the issue.