Pretty much what I said, familiarization training for trainers primarily and for troops acting as OPFOR units at places like the Army's NTC. That being said, it is a bit odd for somebody in the US military to have one in the field, even if they are in the inventory. Are there any Special Forces units stationed in the DMV area or other units that are tasked with training duties that would explain the presence of an RPG?
Thinking of it, the bigger question is, why is there even something like an RPG being used at all? Things like an RPG, AT-4, LAW, SMAW, etc. are going to be of limited use against walkers since they're primarily anti-armor/bunker buster weapons though they can be equipped with anti-personnel warheads but those would be found mostly with RPGs (but we probably don't have a large stock of anti-personnel RPG rounds) and SMAWs since the LAW and the AT-4 are pre-packaged weapons. A more useful weapon against a horde of people (dead or a live) would be a Mk.-19 automatic grenade launcher or an M203/302, but not regular frag grenades since they rely primarily on shrapnel to kill and don't produce that large of a blast.
