Indeed they do - when I referred to pmUA flights into T1, I meant pmUA routes - doesn't make any difference as to which subsidiary flies it, it's the route that counts. It would be difficult towing aircraft between the two terminals (which are a long way apart and would have to cross a runway), so effectively the return route for an inbound from IAD is limited to IAD, ORD, LAX or SFO. By using a 757, they are effectively limiting it to IAD, I believe.