My mother works for a big retailer in the UK and they have a zero vacation policy around Christmas and it makes perfect sense. Someone has to work, as you say it's the biggest sales period of the year, so how do you decide who does and doesn't work. By saying "everybody works" it removes that issue entirely.
As far as I'm aware that's illegal here in Finland. Businesses like stores hire temp workers and have people work overtime (thou within legal limits) during the Christmas season.