I can understand the questioning of other federal departments but considering that the Forest Service is under the Department of Agriculture, and given that the Forest Service Police is responsible for protection in national forests, and given the amount of illegal marijuana cultivation that goes on in these remote sites, then it’s understandable that they might want to be able to outgun the drug dealers.
My guess would be Forest Service, which is part of USDA and which has a law enforcement component. The Forest Service manages some very large and remote areas in which the local non-federal constabulary may be nowhere to be found, the nearest Dunkin Donut being 120 miles away. Some remote areas are frequented by two legged wildlife considerably more dangerous than bears. Meth labs, smugglers (human and drugs), assorted others hiding out, etc.