They're get-rich-quick schemes plus money laundering/capital-flight mechanisms. And if you think capital flight is more ethical than a get-rich-quick scheme, I'd like to introduce you to X oligarch on Y continent committing Z poorly publicized genocide.

Edit: that said, the way that they facilitate dubious money transfer is pretty what makes them likely to stick around and so one can say with more certainty their value will be maintained. Hope that makes you feel better.