With the rising cost of food prices, I think that Afghanistan could do well concentrating on agriculture.
If the US wants to push ethanol, a robust strain of maize could be a good cash crop for Afghanis which might displace some of their 'other' cash crops.
If all the processing were done locally, prices could be kept low and the ethanol could be used regionally to increase the available gasoline, which is scarce due to a lack of refining capacity even in oil rich countries like Iran who have to import refined gasoline.
There's still the question of moving it, and the Muslim-friendliness of making moonshine even if it's poison, but at least Afghanistan would have a legal commodity with a rising value. If cellulosic ethanol production becomes viable anytime soon, then the maize could be swapped for some other suitable biomass which would further reduce costs.
Wouldn't it make more sense to simply grow food to sell?
Or, if they must grow fuel, biodiesel has none of the Islamic problems alcohol might (though I believe if it's denatured, there's no stricture at all - after all, alcohol distillation was invented by Muslims), and isn't corrosive like ethanol.
Ethanol fuel is pretty much a giant mistake anywhere but Brazil, where it's made from waste.
Sigivald,
I'm going from the assumption that the Afghanis would already plant food crops anywhere they could grow them.
I'm essentially talking about hardy biomass crops that would grow where more edible ones might not.
Turning that biomass into diesel is obviously a good idea too, if those crops can be grown. (I think the oily plants require better conditions)
Because of the lack of infrastructure, anything they're looking to export should be very "value dense" to make it profitable. Fuel of any kind meets this condition.
Diverting existing land to producing ethanol or biodiesel is, as you say, a giant mistake. But if you're cultivating unused land or using waste, then it makes more sense.