Vulture funds buy up sovereign debt issued by poor countries at a fraction of its face value, then sue the countries in courts - usually in London, New York or Paris - for their full face value plus interest.
Furthermore, in the context of western exploitation/interference with these countries' economies, many of whom have not had recourse to a judicial process for reparations - your notion of what is rightful or fair is perhaps too narrow.
Well face it, it either happens in the West or is not happening at all. And it's not like some obscure footnote, the arbitrage conditions are fully disclosed beforehand.
And also that delicious interest. You forget that the lenders already intend on getting something out of it. The reason they are getting interest at all is that there is a risk. If there was zero risk, there would be no need to pay interest.
One important part is that if I were to give you 10$ in 10 years, it has less value to you than me giving you 10$ now (which is the difference between lending you lending to me and you not lending to me if you were 100% sure I would pay back).
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 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.