Your last sentence is actually the point - and for those who celebrate this religiously, one they should take in account.
Why can't people say what they want and accept it in the spirit intended? "Happy . . .", "Merry . . .", whatever? Those who make the biggest deal of it are, or so it seems, those who have strayed furthest from perceived goals; converted what should be a time of comraderee into one with political and social landmines. From "fa-la-la" to fanatic, perhaps?
Unable to see the forest because of all those trees in the way, bedecked or otherwise.
Why can't people say what they want and accept it in the spirit intended?
"We judge ourselves by our intentions and others by their behaviour."
- Stephen M.R. Covey
For all others "I don't actually celebrate Christmas, but thanks for the well-wishing!" is a valid answer, which can be summarized as "Thanks!", optionally followed by a "You too!"
I'd do the same if anyone ever wished me a happy Hanukkah.