It wasn't enacted by the 8chan community, it was enacted by the shooter only. And endorsed, well, I have seen many people on Reddit and Twitter who say that all Republicans are evil and should be killed, yet when Steve Scalise was shot, they weren't banned for their support of terrorism and assassination. The same principles you apply to your side should be applied to the other side.
It was enacted by a member of the community and other members of the community voiced support for the acts before, during, and after the attacks.
> I have seen many people on Reddit and Twitter who say that all Republicans are evil and should be killed
You can find anyone saying anything anywhere on the internet, but it's very obvious to anyone who has actually used 8chan that it's a particularly toxic community that is generally friendly to violent ideologies.