Tricky situation, though, for flights into American airports. I suppose they could allow the passengers off the plane, where they would have to be processed through immigration, clear customs, (would they be required to retrieve their checked bags?), and then exit the secure area, only to have to recheck all their bags (?), clear security, and re-board their flight. I could see this easily taking a couple hours under normal circumstances. With an un- or under-staffed station as at PIT? Could take much longer. All the while, conditions might have changed at IAD allowing them to take off. But if everyone is off the plane, you're set back at least another couple hours or longer.

Pick your poison.