I think that's a faulty application of numerical logic to language- it is either used because of region, or possibly education, or emphasis, but I cannot agree that the conclusion of 'I ain't got nobody' is to smile and congratulate the lucky person. One case where negatives confused me was the Bob Marley song 'No woman, no cry', which I thought meant 'don't cry though\if you haven't got a woman', rather than 'don't cry, woman', which was what Mr Marley meant. ;-)