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I don't think fog lights are considered to be headlights? This page:

Headlight Use Laws for All 50 States | YourMechanic Advice

says:

South Carolina
Headlights

• Headlights must be in use from 30 minutes after sunset until 30 minutes before sunrise.
  
  
• Headlights must also be turned on whenever windshield wipers are in use.
  
  
• Headlights should be turned on when you cannot see at least 500 feet in front of you.

 

I think you would be wrong, given the specifications for headlights, I think they fit, and they count as low beams in the total count of forward facing lights if they are not yellow, but whatever, I've never been stopped, nor have I seen anyone else stopped when running like that.

 

I remember it not always being so.
Improperly aimed, too bright aftermarket installs probably ruined it for the rest of us.

 

Around here when you encounter oncoming traffic legally any light other than low beams is to be off,
Sadly it seems to be popular to fit your high seated transport device with 100,000 watt “spot” lights in the fog position, aim them toward the trees and leave them on 24/7

 

My 2016 Ram can run fog lights without the headlights on. I rarely use them, only if it's actually foggy. I get blinded by the "fog"/"driving" lights in conjunction with headlights, and don't want to blind oncoming drivers. These days, even headlights only is too blinding with many vehicles. Especially when topping a hill.

 

I turned them on in the rain to counter the aspect of the abyss that asphalt can take on at night.
Since I couldn't turn them on without the headlights, i couldn't say if they would help in the fog. The last car I drove in which that was possible was a '90s era Grand Prix, and its fog lights were actually yellow.