I am sure it's down to the whole 'don't use foglights unless visibility is below X' thing which you mention in your other comment. People can't remember exactly how far X was, but can remember that they were told that lights in fog are for Desperate Last Resorts only, so they don't use them.

I've never been able to work out why using foglights if it's insufficiently foggy is such a terrible thing to do. I used to be very wary of using them but when I lived in the boggy flats of Cheshire, we used to get such ridiculous quantities of fog that I decided I was just going to use them when I thought other people would be more visible if they were using theirs, which seems like a better common-sense measure to me. Nobody has ever told me off!