I can at least understand why Beehaw did. They defederated from lemmy.world at the beginning because they take moderation pretty seriously and couldn't keep up with the sheer size of Lemmy.world. Hexbear is simply too active within their fediverse to keep up with all the shit that comes along with Hexbear. Even if it isn't users from here being a bit spammy (I'll be honest, it sometimes is though, it was the same when Chapo was on Reddit) the responses to Hexbear users just have a lot more vitriol than is worth introducing into a space like Beehaw.
Beehaw is very cool about applying for alt accounts as long as you haven't been banned and that's one thing good about the fediverse. You are allowed to go over there, but you have to follow their rules. They don't really fuck with instances with open sign up because of how seriously they take moderation there, so honestly dropping Hexbear pre-emptively is fine. They're pretty consistent with how they federate and defederate around their ability to moderate everything.
Idk, I think with federation we have to understand that Hexbear comes with a huge open sign up userbase with controversial opinions. And particularly active commenters compared to the rest of the fediverse. There are some instances we simply are not compatible with.