Collecting relevant aspects of DU stupid into one post:
National Guard - controlled by the Governor of ND, not the Feds;
The tribe has told those in camp to leave - the governor was aiding the tribe, not Trump.
The camp is being cleaned up in advance of spring flooding season. - The tribe doesn't want any deaths or injuries due to the flooding.
The camp is being cleaned up in advance of spring flooding season. - The tribe doesn't want the rivers/streams polluted or bridges endangered by flotsam during the flooding.