It’s not just about blocking it for yourself, though. Content from all federated instances ends up in YOUR instance’s All tab. So when extremist instances sneak in and start federating with everyone, their shit gets broadcast to a huge audience that everyone can see, including users not logged in. So by not defederating, the admins are effectively supporting the platform of extremists and not enforcing their own instance content rules through inaction. This is what I mean by admins needing to maintain their own standard of content on their own instances.

We just did this at lemmy.one with exploding-heads for the same reasons.