How is this a dog whistle for white supremacists?

When people loot, other people will shoot them to prevent their livelihood from being destroyed. It’s a good phrase to sum up that there is going to be a violent reaction to looting.

Oh, please. Walter Headley is so obscure he doesn't even have a Wikipedia page. The closest he's got is a page for the phrase "when the looting starts, the shooting starts", which wasn't created until after Trump had made his Tweet.

The idea that this is a well-known racist catchphrase which anybody had heard of a month ago is absurd, and you know it.

(For the record, I despise Trump. All I ask is that we keep our criticism of him tethered to reality - it's not like we don't have enough non-fictional reasons to oppose him.)

I would tend to agree but him retweeting a video of someone yelling "White power", clearly and distinctively multiple times does tend to lower the tolerance for any type of catchphrase that has a racist past.