This question is related to When someone asks how are you, are you supposed to answer, "Good," or "Fine," and ask back?. There, the answer by z7sg Ѫ claims it is sometimes appropriate not to reciprocate when asked "How are you?". I guess the following situation would fall under the category of "being asked by a shop assistant":
A little while ago I was visiting a US agency in a European country. There was an American man at a counter serving people. Each time a new person came in, the man would greet him/her with something like "Hi, how are you?" and a smile. Most people responded with just "good" or "fine". When my turn came, I responded with "I'm fine, thank you. How are you?". I could see the man was a bit surprised, but quickly recovered and replied he is doing fine.
When someone asks you "How are you?", he is probably not genuinely interested in how exactly are you doing. Why is it then not appropriate always to ask the same question back, or is it? Can it sometimes be too friendly?