The leader of the United States encouraging law enforcement and the military to shoot American citizens for looting, that's the line.
Which I agree with to some extent, you're not innocent of a crime because you convinced a person to harm another, just because you were too cowardly to get your hands dirty yourself.
But the US is rather famously not British, so I'm not sure if it's a relevant thing to add to the discussion.
Briefly: speech which both incites imminent lawless action and is likely to produce such action fails to be protected as US "free speech".
In this case, my IANAL analysis would be that the tweet had imminent application, but would be unlikely to produce action, for reasons given in https://news.ycombinator.com/item?id=23347453 (tl;dr lethal force is the last resort of a well-regulated militia when restoring public order)
* and am therefore fond of "On the fact..." https://www.cs.utexas.edu/users/EWD/ewd06xx/EWD611.PDF