They're not complaining about foreign joint-ventures. The Economist, of all publications, knows that they're essential to do sophisticated drilling in developing countries.
What they're complaining about is that the proceeds are being directed through private hands, as opposed to the actual governments of the developing countries that need to drill their way out of poverty - for example, how the group is dealing with and paying Zimbabwe's secret police directly for access to the country's diamond mines, as opposed to dealing with the actual government (who may be less friendly).
The government of Zimbabwe is run by Robert Mugabe. It's pretty hard for even a secret police force to be worse than him.
Moreover, the governments of many of these countries are extremely rickety and unstable. They are in the main kleptocracies and certainly not capable of running any indigenous drilling operations.
So, it's not necessarily obvious that involving local politicians/warlords to a greater degree would produce better outcomes.
What they're complaining about is that the proceeds are being directed through private hands, as opposed to the actual governments of the developing countries that need to drill their way out of poverty - for example, how the group is dealing with and paying Zimbabwe's secret police directly for access to the country's diamond mines, as opposed to dealing with the actual government (who may be less friendly).