absolutflying,
From what you have written it seems you have been misinformed. You can in fact use the Arrivals by United facilities at your destination on a First Class award ticket. I am not sure however that when your destination is the place from which you began your itinerary, whether Arrivals service is offered, regardless of how the seat was acquired.
In reading your post, a couple questions came to mind. Are you in fact thinking of Arrivals facilities in NRT? (these do not exist in the airport to my knowledge). To confirm, you are not talking about the First Class lounge prior to departure from SFO? That is also available to you on an award ticket.
Mike
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