Yes, that is correct... the DOT does not have jurisdiction but you are free to sue the TA. Any TA with half a brain would just refund you rather than deal with the complexities and cost of a lawsuit since they are not footing the bill anyway, they are just giving back the refund that they got from the airline.

What is interesting though is that even though I bought my ticket from an OTA on my CC the line item has the airline name on it. So, my guess is that the Airline gave a percentage or fixed fee to the TA for their service.