It's ridiculous to me that a company can say, "Hey, where you live requires less pay for a baseline standard of living, therefore we'll pay you less" but they would never, ever consider saying, "Oh, you live in a town with fewer opportunities and less pay? Well, we'll charge you less for our product then."
Oh, I totally agree that companies do it. Where they choose to differentiate in price or pay is based on their whims, so it's not a question of fairness, IMO.