I have many years of retail experience and I disagree.Second, no one actually finds "Merry Christmas" offensive, at least not in any substantial numbers.
Most people like it well enough, but the ones who get difficult are a big problem. Some are even Christian SJWs who will get offended on behalf of others. For big retailers depending on barely trained help "Merry Christmas" is a seriously losing proposition.
I like my little store because I know so many of my customers. I know who it's a good idea to say what to. One of my regulars is a Jewish woman named Joan. She and I always get a giggle out of wishing each other (the jew and the atheist) huge Merry Christmases.
Tom