The baggage re-check counter just before the IAB exit at Dulles can be used by any airline. Presumably because of the costs of staffing it and handling bags there airlines other than United choose not to use it.

So AF transfers to DL don't get to use it (AF brought 793 passengers through Dulles IAB yesterday). Similarly BA transfers to AA don't get to use it (BA brought 703 passengers through Dulles IAB yesterday).