My opinion is this: I cannot be expected to know the personal beliefs of everyone I meet. I don't necessarily assume that everyone celebrates Christmas, but I assume that well-wishes are to be taken for what they are - which is well-wishes, so I wish everyone a Merry Christmas. I have no particular problem with "Happy Holidays" but it just sounds bland to me.
Christmas is celebrated as a secular holiday by some and a religious holiday by others, so to me wishing "Merry Christmas" has no particular religious overtones.
I would not be offended if someone wished me a happy day on a holiday that I don't celebrate myself.