It's possible the cashiers are sick to death of being put in the middle of this idiocy, and now they refuse to acknowledge any holiday at all. If that's the explanation, I can't say I blame them. None of this is their fault.
Perhaps there is the commonality of repetitiveness but it is Christmas Eve and shoppers help keep cashiers employed. “Thank you, you too” is more personal and just as convenient as saying “Have a good one.”