How about this;
The cops know Trump has their backs and the cops know the Trump supporters are more badass former marine types than the pussy hatters and the cops have given US an opportunity to kick some ass for a change.
I'm not so sure they were "told to stand down" rather, sit back and see if they (we) can handle things ourselves.
If it goes bad, THEN jump in, but give THE PEOPLE a shot at shining.
THAT'S what I think and I hope I'll be proven correct in time to come.