People say "Happy Thanksgiving", "Happy Halloween", and so on for all the major holidays when you go out near the holidays. December 25th is Christmas, even if you don't celebrate Christmas it's the Christmas season (Christmas lights, trees, caroling, signs, decorations, etc). If you don't celebrate it, simply don't say it back to the person.
If someone says "happy Halloween" to me, do I get upset and complain because I don't celebrate it? No. I say it back to them because it's the Halloween season. I don't trick or treat or do anything to celebrate Halloween myself, and I have no problem with it being said or saying it back.
People love to complain about anything these days.