By accepting remote work, they open the candidate pool to the whole world. You compete with people having potentially much lower living costs.
Why would they pay "market rate" if they are in a city with high rates?
I don't take location into account at all. If you want to save money and live in the middle of nowhere then go for it. You should have the exact same pay as the person in SF doing the exact same thing as you.
More vague work can be trickier, but I still think home working can work for a lot of people...