The charge is sometimes made when it is socially expedient or personally convenient (i.e., gifts are involved) the unchurched go with the Christian flock as fair-weather Christ(mass) fans. In fact some Christians make the claim- sure you tell yourself it's secular, social and mere family-based merrymaking but deep down you know it's about the birth of Christ.
In the summer we ritually sacrifice an actual human being to the Aztec sun god Huitzilopochtli- don’t get me wrong! We know there isn’t really a sun god who needs his blood replenished with human sacrifice but well, its just been a tradition so long it wouldn’t feel like summer without it. I guess what I’m saying is Merry Christmas atheist pals!