For years, American taxpayers have subsidized ethanol production in a bid to boost the nation's energy independence.Now a boom in ethanol exports is drawing fresh criticism of that tax credit, which cost taxpayers $6 billion last year.The United States this year became a net exporter of corn-based ethanol, exporting more than it imports, as a spike in the price of sugar-based ethanol made by Brazil has given U.S. ethanol a leg up on prices.