It's a very weak case. The US has strong free speech protections. You would need a pretty clear case of incitement to bring charges, let alone get a conviction. I can't recall the last time someone was even charged with incitement.
Probably the strongest example of potential incitement in recent years was when Mike Brown's stepfather screamed at supporters to "burn this bitch down!" while speaking at the scene of which turned into a riot. Police looked strongly at charging him, but ultimately did not. That was a MUCH stronger case because Louis Head (Brown's stepfather) was at the scene of the crime and had a great deal of influence over his audience, which followed his direction. If he had urged calm instead, a lot of people likely would have followed his direction, although that would not have guaranteed that some people might have rioted anyway.
In Palmer's case, he's not referring to a specific time or place, he is not especially influential on the people who were already rioting, he was more cheering on than inciting. If you charged him, you'd basically have to charge every knucklehead on Twitter, which simply isn't feasible. While I certainly do not like statements like the ones that Palmer made, and especially that Head made, speaking as an American, I do prefer that our country err strongly on the side of free speech and I think it would be a slippery slope if we started restricting speech more in this country.
https://www.cnn.com/2014/12/02/justice/ferguson-protests-investigation/index.html