In the abstract: if I own the infrastructure and someone uses that infrastructure to hurt someone, that someone who was hurt (or the parties who protect them) are going to come to me asking questions. If I just say "I don't know" and the law doesn't protect my willful ignorance, I'm at best enabling harm; I'm at worst socially or legally liable for negligence.

In the abstract, the systems of human governance recognize harm and seek to mitigate it.

So if I'm peered to a network using me as a bridge to do harm, I can't trust that network when the bad starts to outweigh the good. If I can't establish trust via human methods, I'm gonna cut that network off to protect myself.

(The Internet started as people who had working relationships with each other and grew out from there. Even though the web of connections is much larger and more indirect now, the whole thing is still at its core a human construct and beholden to human standards of conduct, because humans ultimately have their hands on the various plugs that are yankable).