I don't think that's true, actually. You aren't responsible for things that can't be reasonably foreseen, usually. There are a few strict liability offences in criminal law, but libel isn't one of them. We don't make everything strict liability because it would stifle people's lives.

I don't think a reasonable person would have expected this outcome, so the owner of the bot is off the hook; though obviously _now_ it's more more forseeable and if he keeps running it despite this experience, then if it happens again he will not have the same defence.