When you sign an employment agreement, location is part of that agreement. If you decide you want to work in a different location than was agreed upon, that’s you indicating you want to amend the agreement.
Facebook isn’t saying: “you have to move and these are the terms”. Facebook is saying, “if you want to move here are the terms”. If you don’t like the terms you can always stay put or find a new job with terms you prefer.
Remote worker here: my home city was not part of the agreement. I don't know of anybody has "and I will only live out of X city" in their employment agreement.
If they're changing from office-only to remote, perhaps it's worth revisiting their contract, but we can't pretend that their place of living is incorporated into it somehow.
That said, your state and possibly your county/city probably need to be known due to nexus (state incorporation and tax issues) and personal taxes (potentially all locales).
I believe this is why FB says they will crack down hard on people who fudge this info — getting in trouble with the tax man could open a Pandora’s box.
I work remotely and my employment agreement actually states that "The workplace is determined in <my address>" in the first paragraph. Might be worth checking yours.
Facebook isn’t saying: “you have to move and these are the terms”. Facebook is saying, “if you want to move here are the terms”. If you don’t like the terms you can always stay put or find a new job with terms you prefer.