I think historically atleast here, only one rear foglight was allowed, i seem to remember the reasoning behind it was, that if there were 2, people could mistake them for brake lights.
Also, on previous cars i owned, it was practical to only have one foglight, as the identical position on the opposite rear cluster, was the white reversing light.
Nowadays it seems a lot of newer cars have 2 rear foglight, maybe this has to do with the fact that nowadays, cars usually have 3 brake lights and are not easily confused with the rear and brakelights.