That's just not how society works. All of our laws are fuzzy and open to interpretation. Everything is uncertain. But just because we can't robotically and unambiguously define all undesirable behaviour doesn't meant that we have to allow all of it. We don't have to let everyone do whatever they want in the comments section of every website online. We should do with online undesirables the same thing with do with any other kind of undesirable: shun them and remove them from polite society.
Automatic moderation won't be perfect, but neither will human moderation. I, for one, would be super happy if the bottom half of the internet were readable again.