I was wondering this too.

TAs, AFAIK, are able to do this, but it’s not within IATA protocol for sure, and violates the agreement with the airline to issue tickets on their stock. Airlines especially don’t like it since typically, TAs use the air carrier merchant accounts to run the charge, so not only is the airline not really getting (or at least keeping) any revenue since the fares go to the operating carrier, but they lose $ because they are paying to process the cards.

Most TAs won’t do this because, kind of like HCT, do it once and the airline probably won’t really do anything. But its probably pretty easy to find, and do it semi-regularly and there will be consequences - in this case likely fines and if it gets bad enough - flagrant and frequent, they probably will lose the ability to sell tickets using their stock.