I think it was to avoid violation of the posse comitatus act.
I don't think that would hold up in a legitimate court. I'm pretty sure you can't take an entire military division and say they are now also police officers. That being said the Republic of the United States ends in two days so anything goes.
We the people just need to understand if anything goes in DC then anything goes in our individual states. Including passing legislation saying federal laws, edicts, orders and rulings are not valid or binding on anyone or any entity in said state. Then everyone in the entire state can stand up face the proper direction and give our new middle finger salute to DC.