What good is that if you're an "at-will" employee? They can just change the rules at any time for any reason and fire you if you don't agree to them.
I have enough experience to know absolutely nothing about me employer is guaranteed/permanent in the US.
Relevant story for the HN crowd: I worked for a small company (~250 employees). Real chill place. Six months after I start they were suddenly bought out by a huge multinational corporation. First order of business is everyone needs to sign a non-disclosure agreement and an agreement that the company owns all the code you write while employed with them, even if you do it on your own time. There were some people who were reluctant to sign, it was made extremely clear that if they didn't sign they would be unemployed.
That one didn't bother me, I only code at work anyway. What did bother me was at another employer right after I became eligible for the employer's (generous) match on my 401(k) (after 1 year employment) an email went out a few weeks later saying "Sorry, we won't be matching 401k contributions anymore."
I know a guy (very senior engineer) who negotiated that he would be working from home 3-4 days a week before taking a new job. Employer agreed. Few months later line manager notices that employee is not logged into whatever work chat they use in the middle of the day. Not habitually, once. Next day it was announced that there will be no more working from home.