Just my opinion but it boils down to whom the State Dept. and the big money boys want protecting them and what the latter wants, the latter usually gets. Some civilians in country may not want a close relationship with the military for any number of reasons, legitimate and non-legit. Since former military significantly fill the ranks of mercs, it is out of character for years of training and experience to be tossed out the window in a firefight as if they were gunslingers in town on a drunken binge. No doubt mercs complicate and impinge on AOs but non DOD covert ops piggyback on mercs from time to time too, they would have to.