YY, I did the same. Let's face it: the incident doesn't have any news value, it happens every day: person X breaks a rule she is not aware of, those in the know (proud of their tolerance) don't say a word for a while, X continues to break the rule, patience wears off, insults follow ("Do you think you can do what you like because you are young and American?"). Ignorant? Rather lacking "soft skills". The right thing to do is to inform the offender about the rule when your patience begins to wear off. On the other hand, a stranger should try to learn as many of the rules as possible, and ask if in doubt.

Interesting about the special case: if an insult is required, old ethnic clichés can be warmed up any time, in conjunction with the ever-acute generation gaps.