It sounds like you are describing new user experience.
And I understand, coming from Reddit, how this can be a shock.
However, that's how Lemmy works.
Similar to how twitter users got a shock moving (or trying to move) to mastadon.
The very nature of the fediverse works better with more instances, where a single instance has fewer users and the communities are more focussed.
Beehaw hasn't "blacklisted everyone from...". They've defederated. Whilst it may seem similar, it's more nuanced. And that's what a lot of people don't understand.
Block-listing all users from lemmy.world from interacting with beehaw would be an amazing ability. That would put beehaw in a read-only state for users on lemmy.world, whilst still allowing beehaw users access to lemmy.world.
Unfortunately, the current admin/mod tools do not allow for that. And manually dealing with the huge influx of toxic users (posting death threats, illegal porn or trolling) was taking too much time.
Besides, the lemmy.world admin is working on custom tooling to deal with this issue. Because it is their users causing this issue, and it is their problem. And there is no higher authority - there are no Reddit admins to say "stop brigading".
Shitjustworks, last I heard, weren't responding to communication.
I have no doubts that beehaw will refederate as soon as Lemmy.world sorts their mod issues, or the Lemmy framework allows for more nuanced mod tools.
You have to remember that Lemmy is young.
It's been around for a few years, but the shear scale of what is happening now is less than 2 weeks old