The interesting bit is that the debt buyers are making money from some advantage they seem to have in enforcing repayment relative to the original lenders.
Under proper rule of law, the original lenders would have exactly the same chance of getting money back as Goldman, eliminating all need to sell the debt at a massive discount.
There is nothing illegal in this, if that's what you are suggesting.
There are different reasons why somebody might decide to sell debt at a discount. One reason, for example, is that bankruptcy proceedings, especially international ones are super messy and expensive. If you are not dealing with these things on a regular basis, it might be better to book a small loss now and get and immediate cash flow instead of waiting for decades.
Under proper rule of law, the original lenders would have exactly the same chance of getting money back as Goldman, eliminating all need to sell the debt at a massive discount.