The collapse of the oil market in late 2014 and early 2015 has had many predictable consequences: cheaper prices at the pump; pain for oil-exporting countries such as Iran, Russia and Venezuela; and a growing bonanza of deal-making as private equity firms snap up distressed oil producers.

But perhaps one surprising effect has been a revival of the long-standing debate over the impact of speculation in commodity derivatives. As in 2008, when skyrocketing prices caused widespread public criticism