The D.C. Circuit Court explained clearly in the Park vs. Commissioner tax case, why the IRS was wrong in its determination of U.S. tax liability of non-citizen gambler winnings.
■US Court Soundly Rejects IRS Withholding Tax Practice on Non-Resident Gamblers, July 18, 2013 http://www.procopio.com/news/us-court-soundly-rejects-irs-withholding-tax-practice-on-non-resident-gamblers