My guess is that the OP saw the charge, called United and asked what it was about, and United told him the exacting routing from LOS-IAD-LGA and the names of the passengers for whom he had supposedly bought the tickets. The names were Nigerian. I don't think he's assuming anything.

If my card was used for a fraudulent purchase of plane tickets from Dublin to Boston in the names of Seamus O'Riordan and Siobhan Barry, I would reasonably conclude that Irish people traveled on my dime, I don't think that would be an unfair racist assumption.