I personally imported a Golf GTI MkI in '83, which only had one rear fog. Popping the light cover off revealed no bulb in the other side - fitted a bulb, and hey presto, two rear fogs! Clearly the cost of a bulb is significant when you're building tens of thousands of cars. When I replaced it with a Quattro coupe a year later, it also came with just one fog - but investigation of the rear light mounting PCB revealed a more complicated fix, so I went with just one.
I suspect the Abarth/Mazda electronics are a whole lot more complex now so doubt there's an easy way to grow another one.