I'm a little confused of the role of the PMC. In the description it sounds like they can go either way depending on the condition. But realistically you wouldn't see Black-water, L.I.O.N. or any other PMC "sanctioned by the UN" open fire on US troops or UN personnel.
well, that's because they can go both ways. If you read the back story they are a corrupt PMC, that is exploiting the situation to make money. Its just a way to exploit the story to make an interesting aspect to the game.
Realistically, no, you wouldn't see any "UN sanctioned" Private Military organization opposing US troops in real life but this scenario is entirely a work of fiction. While the 82nd airborne is obviously meant to represent the real 82nd airborne, and the OLA is a generic gorilla army, the PMC are in no way meant to reflect any real PMC, such as Blackwater exct. who do a dangerous but noble job protecting assets in really messed up places.