Frontcheck
I am really sorry, but I don't get the thrust of your last post.
A passenger who asks for a schedule change, that is not permitted within the fare rules is asking for a variation of contract.
The airline is at liberty to accept or reject that request; if it accepts the request, it may choose to levy a charge or to vary the contract free of charge.
There is no point of law that forbids a customer requesting a variation of contract and if one set of employees are overruled by another set of employees, then that is an internal matter.
A customer is perfectly at liberty to escalate the request and in Captonian's example, the personnel at head office decided to vary the contract, free of charge.
If the employees are humiliated or frustrated by being overuled, then they are perfectly at liberty to resign.