Lots going on here: Twitter labels this tweet for violating Twitter’s rules about glorifying violence; the tweet stays up because of its “public importance” test; Twitter further disables retweeting without comment so it can’t go viral so easily. RT with comment is OK. 
Then, the official White House account has repeated the words Donald Trump said as realdonaldtrump - “when the looting starts, the shooting starts” - perhaps to associate the full force of the office of the Presidency with them. I imagine Twitter rules will apply again.
Twitter has surely waded into a fraught zone. Good. One of the ways to try to break what is mad online behavior of even a (especially a?) President is to create rules to deal with it - rules that would have no place in normal circumstances. Then endure and respond to blowback.
The President is abusing the baseline deference and respect typically accorded to the platform of the Presidency by adjacent public and private institutions. He wants to be protected by powerful norms while not himself bound by them.
Twitter, in turn, has tempered its deference.
Twitter, in turn, has tempered its deference.
