It's not correct that "nothing else will grow there".
As far as I've understood the problem, nothing else will earn them anyway near that much money which they get from planting poppies. So, they're reluctant to change to something else.
The main problem I see is that we provide the market and "ask" for the product. They just provide and make a living out of it. If we stop to buy they'll need to change. But we're not stopping.
The whole thing about police forces destroying fields seems pure propaganda to appease the foreign forces in the country. The fields are basically not controllable for the forces that are in Afghanistan now.