It's not offensive to say Merry Christmas to someone who doesn't celebrate it if you didn't know. In a country where the vast majority are Christian and celebrate Christmas it's safe to assume that most people celebrate Christmas. Making an honest mistake based on that assumption is okay, just remember from now on. So long as you acknowledge and respect people's traditions and beliefs, who cares? Then again, I don't go around saying Merry Christmas or Happy Holidays to strangers either, so I suppose I avoid the situation entirely.