When I read about non-ideologically driven ("market") insurgents, the first two things that popped into my head were mercenaries and pirates. They still exist today, of course, but not nearly at levels seen in previous eras.

I think there's some ethnocentrism here as well. One person's "illegal" or "black" market is another person's legitimate enterprise. The opium trade in Afghanistan is but one example. If Afghanistan produced widgets instead of opium, the Taliban and other groups would get their funding from widgets just as easily. What would John Robb's argument be then?