As the commercial used to say, "How do you think they got so rich?" Wal-Mart Stores Inc. owns more than $76 billion of assets through a web of units in offshore tax havens around the world, though you wouldn’t know it from reading the giant retailer’s annual report. A new study has found Wal-Mart has at least 78 offshore subsidiaries and branches, more than 30 created since 2009 and none mentioned in U.S.