As I have read, the 'troops going to Portland' have been federalized and their role is only protection of federal assets/buildings. They cannot do police work.
Putting troops on guard on those buildings does, however, free up some of the police and other agencies who were guarding their home base buildings to go out and do the 'chase down illegals/druggies/gangs' task.
I have a problem with soldiers doing police work. Military police are bad enough at doing police work, putting an infanteer in that role is - un-smart.
Putting an infanteer on guard duty around a building so that the DHS or whoever can safely ingress/egress from their building and go do their work in the public space as a peace officer/police officer makes some sense.
It's certainly inflammatory, but it makes some sense.
Of note - I'm not a legal beagle on this stuff, and I'm able to accept that I'm wrong if that's the case.