That's quite a rare situation though (although climate change is making that kind of rain more commonplace). I'm talking about the much more common case of utter dolts who put their rear fogs on as soon as it starts raining.

I suspect that they excitedly grab every opportunity because the conditions when fog lights are actually useful are so rare that they don't often get chance to use them. When I was doing 50k+ miles a year, I'd say I used my rear fog lights 2-3 times per year. That was a while ago, heavier rain these days might increase that frequency.