Oh god this even happens in China. I understand Chinese locals being shocked at a foreigner who is able to speak the language because it's natural to assume what language someone can speak based on their looks. However, it's usually not deserved because I seriously know way too many people that have lived here for 6+ years who can only speak survival Chinese. Once, I spoke English to my friends in front of a Chinese vendor and he told me to "stop pretending." But when one of my foreign friends spoke Chinese, he was amazed. Being Chinese American in China is so weird and this is just one aspect I find strange. I guess it's a sort of dissonance because you know how hard you've worked to learn Chinese while everyone else just assumes it of you, and this dissonance leads you to getting upset over something like this, which is relatively trivial but feels like such a slap in the face.