Ok so you agree that in my example you are stuck. I would imagine that most people...myself included as i do always do it, would say to an agent..
Can my ticket be rerouted mid trip?
They say 'Yes it can'.
So. to use my example, is it then allowable to say to me when i am in LAX...actually sorry now the conditions are such that actually you cannot change the ticket any more to return from PHL. you can only change it to SAN or DEN etc that has a higher fare available. Ie we will dictate as to where you can fly back from, even though everywhere is available. Do you honestly believe that BA would say this is correct and not allow you to return from PHL?
In contrast, and in utter ridiculousness, had I have bought a cheaper ticket then this would be no problem.