why, because this is a good example of an incomprehensible fare rule. My question, which you ignored, is "what is the charge to change the return flight after travel has commenced?"

Maybe you can see it somewhere in the fare rule. I can't. And this language is pretty opaque to someone like me who is not an expert in airline fare rules.

So the advice "always read the fare rules" does not help many people. what good is it to read a rule that is incomprehensible?