It can't be mining, otherwise we would see similar patterns in South Australia (at least) with similar patterns likely to be visible in every other state as well.
I'd be interested in knowing why this is so as well.
There's similar patterns in north central Siberia (loaded with primary resources) and a couple of other places. There's a weird bloom affect on these spots that's not evident in populated parts of the map. You can even see the bloom effect on the north coast of Alaska, where I'd be extremely surprised that there's light of that level.
My guess is that they didn't quite finish the job off right and got lazy when manipulating these areas.
I'd be interested in knowing why this is so as well.