This is an outside stand, it isn't Joe and Mac's apartment in San Francisco. The humans are waving their holiday in the Na'vi's faces. I would have been perfectly alright with a Na'vi holiday being present instead. But it seems wrong to have the human's Christmas.You aren’t representing the story of the land. Guests are being welcomed to visit facilities built by humans (in this case, a former RDA mess hall). It would be like me inviting someone to visit my lab. My lab has a Christmas tree in December.