Verified answer

If you purchased a full fare or business or first class ticket there is normally no change fee.

The discounted tickets are priced lower to allow the airlines to fill vacant seats at the last minute.

If you change flights this affects the airlines projected costs to operate the flight, so you will pay for the change.

It should be clearly spelled out in the terms and conditions you agreed to when you purchased your ticket.

Those are the terms you agree to when you book the flight. They do it to make more money as a business and to give extra benefits to those paying full fare.

Why? Because it's just another way to gouge the unsuspecting consumer who almost never reads the fare rules or terms and conditions.