The leader of the United States encouraging law enforcement and the military to shoot American citizens for looting, that's the line.
And looting always leads to shooting, regardless of who is saying it.
Even if one views the Tweet as a legitimate military command, which it is not, unless the government has seized Twitter with just compensation as required by the 5th Amendment, it is not Twitter's obligation to ignore its own standard sfor the purpose of relaying such orders by the President.
Otherwise, except as to explaining why Twitter opted for the public interest notice rather than simple removal, the President’s job is irrelevant here.
> If he gives a warning that looting will lead to shooting, it is not glorifying violence.
That doesn't follow from the preceding, and the statement as written glorifies violence, both potential future violence and specific historical violence by the government against it's citizens, in much better the same way (though far more proximate historically and thus much worse) that it would if Trump said “Kill them all, and God will know his own.”
> It is a statement of fact.
It is quite possible to state a fact (or make a threat which one has the power to declare execute, which is more the case here than statement of fact) while glorifying the outcome that would be produced and/or the past historical antecedent which is invoked.
Next, the 4th, 5th, and 6th amendments to the United States constitution. Telling armed forces to shoot unarmed people because they happen to be looting (e.g. when there is no imminent threat to life) is summary execution and unconstitutional.
How about martial law, seeing as we're throwing around unrelated legal laws and concepts?
Either way, these ridiculous "protests" indicate a complete break down of society and the government should be sending in the military to arrest people.