They are still legally bound to deliver a valid reservation. If the airline has cancelled a flight a valid reservation was NOT delivered to the consumer - nothing of usable value was delivered to the consumer - so the entire transaction is void. This is akin to the OTA just delivering a phony reservation to the customer and keeping the money, that isn't going to fly legally.

While the DOT may not be able to help I will file a lawsuit to get 100% of my money back because I do not think the OTA can operate this way. This does not happen in any other industry. If I buy something from an online retailer and do not get the product for whatever reason I most definitely get my money back.