"Really, just because someone is a tourist does not entitle them to assistance from anyone who is passing. If someone rudely stops you and demands you direct them somewhere, you are under no obligation to do so."
Firstly, there's nothing to say this person was a tourist, and secondly, nobody said that he was entitled to assistance. It's just that if she chose to answer him at all, she was obligated to avoid rudeness in doing so.
"Maybe I am too literal-minded, but if someone (rudely) asks a question, and you answer it politely, it isn't rude, even if you did not answer the question the person thought they were asking."
That is being too literal-minded. She didn't answer politely, she disguised a wisecrack with polite words. She could have failed to answer the question at all, answered with "I don't" or "Please stop shouting", or given him directions, and any of those replies would have been polite. What she did wasn't.