In a hypothetical scenario where an Italian affiliate refers an Italian customer who buys a violin on Amazon.it, there's of course no withholding of U.S. tax on commission due. Replace Italy with UK, Brazil, etc. for the same outcome. Delivery address of the referred customer seems to be a key factor.
In what exact scenario did you not tax withheld? Please describe in detail. And just in case, are you a non-U.S. person in the first place?