Holidays aren't static. Holidays serve social and cultural needs, which means that as society and culture changes the manner in which we celebrate our holidays also changes. The "traditional" Christmas people like to reminisce about is of relatively recent vintage, with all of the elements not coming together until the 20th century and many elements not even starting until the 19th century. There is nothing permanent or necessary to the way we celebrate Christmas today and Christians don't have much basis for complaining when changes occur.
One of the most significant changes to Christmas has been the integration of Santa Claus into the holiday. At one time Santa had nothing at all to do with Christmas, especially when it was celebrated as a primarily religious and somber date. In modern America, though, Santa has become the very symbol of the entire holiday season -- and at the same time that Christmas has become secular. This is not a coincidence. The secularization of the Christmas has required sidelining Jesus, creating a vacuum at the center of the holiday which had to be filled. Santa fills this role admirably.
It's probably also not a coincidence that Santa and God share a lot in common: both are supernatural beings which constantly monitor people's behavior so that at the end (of the year, of life) they can reward the good and punish the bad. The punishment aspect of Santa has fallen by the wayside in America, but then so has the punishment aspect of God -- at least in mainstream Christianity. It is for reasons like this that some believe indoctrinating children into believing in Santa effectively prepares the ground for belief in other myths, other supernatural beings, and other nonsense. It is thus argued that atheists shouldn't teach their children to believe in Santa Claus at all and instead to tell them the full truth as soon as they begin to ask.
It is ironic that any effort to argue for telling the truth about Santa should in theory be something that benefits the Christian Nationalists because they shouldn't appreciate the way Santa has replaced Jesus. The reality is that they tend to regard Santa as emblematic of "traditional" Christmas, so any assault on the position of Santa will end up annoying them.
The above image is based upon a World War I poster of a child who is happy that her father bought her a war bond.