It’s common to receive a mail from some service that they’re changing the terms and conditions of their service (internet provider, gmail, etc.) These changes are unilateral and can be about anything, from their privacy policy to the money they intend to charge me.
Why can’t I unilaterally send them a mail and say, I am changing the terms of my patronage and will now insert my new policy (pay only xx amount, or demand a mint Chico chip ice cream every Tuesday)?
If you and I make a contract, and then I turn around and change some terms with crayon, it’s not like you’re beholden to the new ones.
The one who dictates the terms of service is the one who provides the “service”, questionable though it might be. You agree to it when you sign up. When they change it up on you, it is with the assumption you’ll quit if you disagree strongly enough to the changes.