That said, if I wanted to spam the Fediverse, I’d just spin up my own instance of Lemmy or Mastodon.
Its actually smarter for spammers to infiltrate populated servers. Admins aren’t going to have a problem defederating from a pure spam instance. They’ll think twice about defederating from an instance with lots of legit users.
It’s not as easy as moderating individual posts. Remember, Lemmy is decentralized. If you start your own Lemmy server and I federated with it, I’ll get all the stuff you post on my instance too (intentionally oversimplified).
Its up to you to moderate communities on your instance the way you see fit, and up to me to moderate mine. Even though our instances are federated, I can’t moderate on your behalf. It just isn’t feasible both in terms of the technology and in terms of the sheer volume of content you would have to try to moderate.
If you have a community that posts a mix of things I agree with and things I don’t, I really only have a couple options on my end. Basically I can block that community on my instance or block your instance altogether.
The reason why someone might block a community may be more about the legal risk than any moral justification. Depending on where you are, it might be illegal to even host that information. And since Lemmy instances cache posts from other instances, it could be argued that because that community is federated with your instance, you’re responsible for the content posted there.