Well, the issues are defining limits of what constitutes as the limits of "harmful" in free speech.
The age old anecdote of yelling fire in a crowded theatre applies. Free Speech is a double edged sword - make outrageous and potentially criminal claims? Prepare to own up to it.
Vagueness is the greatest danger and strength of free speech. And unlike like many European countries its enshrined in our policies, even if our government does a terrible job at administering it.
This isn't a swipe at you Euros, but my government, at least at the onset, understood that a robust protection of basic dialogue rights was needed. Various legal cases, court battles, etc have established a clear requirement of establishing that "harm" actually has to have strong evidence showing the accused seeks to carry out on their threats, not merely to incite.
As for the kook who spun #killallwhitemen? Well, this should be a cautionary tale of special interest groups demanding the right to define what is and isn't harmful speech. The crazy RadFems want the exclusive right to determine which speech is harmful or not, but I find it pretty great the methods they're trying to use can and will be deployed against them as well.