I don't mean to offend, but... you had round trip tickets and you successfully used half of the round trip tickets. USAA refunded the half that wasn't used.
The airline f'ed you over and charged you for a one-way flight, and you paid with USAA thinking you could get a refund of that one-way ticket after having flown back. USAA said no, you flew on that ticket, we can't charge-it-back to the airline.
You're dispute is with British Airways, not USAA, which appropriately refunded the merchandise to which you paid for, but did not receive. British Airways can prove it was in fact, you that bought the ticket, and you did in fact, fly back home on the ticket. A charge-back would not be successful and USAA would be on the hook for $2,000 that you voluntarily paid.
If you had paid for travel insurance or something that covered itinerary changes on the part of USAA, or if your terms specifically covered that, then I would agree with you.
That's not to say it doesn't suck to be f'ed by British Airways, it's just to say in no way is any of that USAA's fault and they did appropriately refund what they could, and it's not implied to trust that your card company is going to refund a ticket you paid for any used. [sorry]

