Well, one of the disadvantages of an OTA rather than booking directly with the airline, are fees like this. OTA fees for refund of tickets - even involuntary refunds - are quite common; some OTA will apply a fee, some will waive it in a situation like this.
Did you check the terms & conditions of the OTA on the webpage? I bet it is stated somewhere in the small print part that the OTA can/is entitled to charge a fee.

Most likely no grounds for dispute and need to move on - next time book with the airline directly.