So?
Is there a law in the land that states specifically that you cannot put up holiday displays? Seeing as the vast majority of this nation does claim to be some sort of Christian denomination, why not let this occur? It's celebrating "the reason for the season", or so the billboards tell me.
Now, if the local government actually block Jewish, Hindu, Muslim or whatever displays when the request is put forward to do so, then they would be supporting one religion over another and yes, you would be correct in saying that they are "promoting one religion over another"