For one, Hexbear made it perfectly clear they expect their users to abide by the rules of the instances they visit and not cause trouble.

The users at Hexbear made it perfectly clear in the discussions about federation that they weren't going to respect that request and were openly encouraging each other to brigade other instances. They seem to have quite a number of participants who were really looking forward to being able to "bully the libs" and a quick scan showed several comments about sending offensive DMs to users of other instances. I was also able to find multiple HB users who openly admitted to already being banned multiple times from beehaw for trollish community breaking behavior.

The idea that BH users would have been well behaved is wishful thinking as they've already shown the Fediverse how they behave. Their instance Admins and Users are also not shy with how they are approaching Federation. They very specifically want the freedom to participate in other instances without those instances being able to engage them. Put together they want the freedom to "own the libs" all across the Fediverse without having those same libs show up in their carefully crafted safe space. They want to eat their cake and have it too.

To be clear I don't agree with how Lemmy.World handled the blocking of Hexbear. I would strongly have preferred community engagement on the issue prior to the block but looking at the Troll Army being formed at HB I can't necessarily fault the admins of LW either. Dealing with ideologically motivated extremists is a lot of work that no one wants to do.