To be clear, I'm not trying to say it's unfair for atheism communities to have those rules. Your community, your rules. But if I'm seeing those posts, I'm going to be bothered, and if I respond, I'm going to break the rules and bother everyone else. Better to not see it in the first place.
In the same way, I'd expect atheists to block a Christian community if it were trending - though I'd be willing to meet them in constructive debate in such a community if the rules permitted it.
Blocking is a community is how I control my personal feed, it's not a condemnation.