Well, "abusive behavior" isn't necessarily illegal, so that's the first point.
The second is that this will absolutely wreck free speech online. Sites will have 0 incentive to not be overly aggressive in eliminating anything they think might theoretically piss off the authorities. It's expensive, and will harm smaller communities that don't have the resources to do this effectively. Even YouTube has struggled to address things like jihadi recruitment videos, and that's with the resources of one of the world's largest tech companies.