well, it doesn't have to be a fixie...
It's just that during winter the salts and grime can be a bit abusive to the drive train.
So FG for simplicity in maintenance and getting a feel for traction on the rear wheel.
freewheel can freeze, dérailleur can freeze, the cogs can get caked with ice and might have poor shifting... that's about all winter has to offer to a geared bike.