So there's this thing happening right now about people who are getting offended about how a stranger greets them over the holidays.
Consider this: From November 1 - January 15, there are about 29 holidays observed by 7 of the worlds major religions. "Happy Holidays," is probably the most all encompassing greeting as it means you are aware of all the other beliefs systems out there and want to be as inclusive as possible.
A greeting is personal to the individual saying it. They are not obligated to say what you want them to just as they are not able to magically know what your personal preference or belief systems are. So instead of getting offended when someone greets you, be grateful they took the time to acknowledge you at all. Respond according to your own beliefs and go about your day.