If not a PO Box, some other proxy by which you aren't genuinely living there. Maybe just a short-term roommate lease situation?
It's almost a no brainer to me. If I can charge 3-4x more for the same skills that I have, why not. I'm not gonna take a huge pay cut just because my location changed. And there's almost no explaining involved.
At times when you can't go somewhere physically, sometimes you can tell your clients you're traveling this month. If something is important, you can take a flight. The amount that you're saving, the flight costs are negligible.
Also, what happens to a full-time remote worker who decides to move? If they tell their employer that they moved from SF to somewhere far cheaper, should that employer dock their pay? What if they're just temporarily moving (e.g. living abroad for a year, but returning afterward)? If you're going to work remote, it may make sense to rent for a month or so in SF to be present for interviews and then move back to wherever you want to live, which would essentially mean you "live" in SF for that period, so it wouldn't be lying.
I know that shit works because rich people do it for taxes all the time.