The celebration of Christmas, including the exchange of presents, was spawned by retail business to increase sales, and for no other reason. For the same reason (sales), businesses avoid saying "Merry Christmas" to not turn-off disagreeing customers that may then choose to shop elsewhere. Many other religions celebrate religious holidays in December, so "Happy Holidays" covers it all, without having to ask what that person's religion is and then knowing the appropriate greeting. There is nothing illegal about saying "Merry Christmas". However, a business cannot force an employee to say any religious greeting, unless it's part of their job (like a store Santa). In fact, throughout history, some Christian religions, like Puritans, thought it wrong to celebrate Christmas, other than purely religious ceremony. They even passed likes against anybody working on Christmas day. A business can enforce policy against an employee that persists in using language to a customer that is a matter of religion, that may be in contrast to that customer's own exercise of religion, whatever that religion, that goes beyond the reasonable accommodation allowance for freedom of expression. Saying "Merry Christmas" or "Have a Blessed Day" are just two examples. For the most part, an employer can fire an employee without even giving a specific reason.
