[easygoing]

In this situation you thought your pre-alert of not flying the second portion would be a good faith unfortunately it was not been appreciated by airline as you expected and even they disallowed it
apparently what the airline considered is the revenues which are supposed to be generated by their meticulously designed pricing system and your practice did not accord with this system ,as Mr.Jim said you are not alone in this frustration
airline pricing system or operation procudures may not be as same as what a normal our passengers thought ,I am sorry for the unconstructive advise .
but at least you contacted them in advance and obtained the negtive information just
better than you come across an unhappy surprise when you take your return fly and find it canceled
just hope more customer-friendly rules will be set out and enforced
good luck