It certainly makes sense that airport fees/taxes and fuel surcharges on previously flown flights are non-refundable, but irt flights not yet flown.....? That's certainly not part of any rules that I've ever read.

If you previously were routed to Brazil but are now ticketeting to fly to Argentina instead, it's absolutely crazy that AY would assert that your prepaid Brazilian embarkation tax is non-refundable, and that they are somehow entitled to pocket that prepaid government tax, while now charging you fully (extra) for the Argentina customs and immigration tax. I suggest calling back in hopes of speaking to someone in the fares department who can clarify exactly how the actual calculations are being performed, as opposed to someone who may have been referring to flights already flown, or speaking theoretically (and incorrectly) about the process. Another alternative, if your itinerary includes any AA flights at all, is to initiate contact with the AA RTW desk.