The National Guard is a legal organization in the USA.
It is perfectly legal for the President to order the National Guard to put down civil disorder and rioting, using all force necessary to do so, up to and including shooting rioters, should that become necessary.
What’s NOT legal, “Pocahontas”, is lying about your ethnicity on official applications to gain personal benefit. That is called “fraud”.
That is NOT what President Trump said and it is not standard operating procedure unless you are Chicago's Mayor Daley.
President Trump said they would shoot looters. That is standard operating procedure for the national guard once they are put in service to protect a community which has gone through a disaster.