wingzing, a cancellation of any flight on your itinerary entitles you to a involuntary refund of all unflown segments. Refund should be on a pro rate basis, most obviously based on miles flown relative to miles ticketed. You have a lot of miles left -- HEL-HND-BOS is a long way. Would you want that refund?
I recommend getting AA's position in writing or at least in a recorded phone call which you can transcribe. Sounds like AA is very much out of line in claiming your ticket is now "worthless".
To me this is a legal matter, not a customer relations matter. Agree with anabolism's suggestion of a formal complaint to DOT. Your credit card issuer could also help you get a refund, if that's the remedy you want, based on cancellation of ticketed flights.