Seven years ago, the U.S. led an effort to address a problem facing governments everywhere. Each year, people manage to avoid paying an estimated $2.5 trillion in income tax — a giant sum that could be used to combat poverty, update infrastructure or lower tax rates for law-abiding citizens. Now, however, the U.S. is becoming one of the world’s best places to hide money from the tax collector. It’s a distinction that the country would do well to shed.