Obama's Wife and Walmart

I haven't seen anyone mention that Obama's own wife, as reported by abcnews back in May, 2007, has "come under fire for serving on the board of a company that is a major product supplier to Wal-Mart." Knowing this, why would Obama throw the first shot in the South Carolina debate, and accuse Hillary Clinton of serving on the board of Walmart while he was working for civil rights in Chicago? Seeing how Obama in the debate conflated Hillary with Bill, criticizing her for her husband's comments, why hasn't Obama's wife been mentioned at all?