My counter-argument is this: you could make a more extreme version of the statement like: "<i>the government</i> can't arrest you for what you say, but <i>the people</i> are free to reach for their pitchforks and lynch you". Which isn't true, the government must also protect you from arbitrary violence. The fact that violence perpetrated via social media (harassment, stalking, mobbing) cannot realistically be prosecuted, doesn't invalidate the principle that people should be protected from it.