I say "Merry Christmas" to them because I'm extending them greetings because of my holiday, not because I'm trying to convert them. If they interpret that the wrong way, they're just looking to pick a fight anyway.
Not just that, but Merry Chrismas is at its heart a Christian Holliday, but it has ALSO become an American/Western secular Holliday so wishing someone a happy Christmas Holliday shouldn't offend Anyone!
I agree with you, and that's part of the reason I go around saying "Merry Christmas" to everyone with a big smile. And I am not afraid to get into a fight with anyone, though I don't tend to fight with fists but with cold, clear, British-style words. But I can understand that in a community in which christians are a minority, it might be an understandable decision to tell employees not to have fights with customers. There are a lot of them around here who would sue at the drop of a hat.
"It is as bad to take offense as to give it" -- old Chinese proverb.
How right you are. If everyone look at this from your prospective then there would be no disagreement or wrong interpretation of someone's meaning of wishing them a Merry Christmas.
Sadly though, that is not their goal. Some want to be offended by the celebration of our traditional holiday. Others just want Christ out of everything.