Our office regularly receives inquiries from Canadians who vacationed in the USA where a casino withheld 30% on their winnings and they'd like to pursue claiming a refund. Typically such claim works for Canadians (and not other nationalities with less favorable tax treaties with the USA) whose losses exceeded their gains.
This article is of interest because (1) it applies to a resident of a country that doesn't have the special favorable gambling loss offset clause that contiguous countries (Canada and Mexico) enjoy and (2) the taxpayer did not need to have incurred any losses in order to have claimed entire exemption from US tax withholding on his millions of dollars won gambling in the USA on a sporting event bet. Had the taxpayer simply qualified as a resident of the non-US country involved in the case, his zero withholding claim would have been undisputed by IRS. Instead, in spite of paying a tax to that non-US country, he could not establish that he was a resident of it and lost his claim only on the basis that as a non-resident he couldn't claim the treaty benefit.
An interesting read for US casino visitors from abroad. Click on the above image of the case to view the PDF.