It's because the money they spend is split 50% to writers and 50% to the person who referred (less admin fees). This means there's less for the writers than there might have been if they'd paid through the normal membership route. But the powers that be argue that it doesn't reduce income for writers. It increases it. Presumably because they think more people sign up than would otherwise do so. I struggle to think they wouldn't sign up otherwise, but who knows. I don't have any data either way.