I was trying to use an absurd example to point out that it's not unreasonable to set some limits on what an employer can ask of a (possibly potential) employee.

I was not commenting on what those limits are or should be. I was only pointing out that they exist.

I don't understand how this relates to your own example. You seem to be trying to say that a contract is either completely mutable in an unlimited way or else totally immutable? I don't see how that's true at all. There are many limits on contracts already, and yet they still change frequently as situations change.