Because something certainly seems to be doing so. My personal guess, shared by a friend of mine, is pleospora papaveracea, a fungus that attacks poppies, but leaves other plants alone.
That certainly seems to fit reports from the field - and it would neatly sidestep the central military-political problem created by conventional anti-poppy efforts, while providing a boost for programs aimed at a farmer-centric approach to counterinsurgency.
Well, well, isn't that convenient? Then again...
The fungus is found in India, Nepal, and Pakistan, so it could be natural. Especially given the Taliban's kick-up of cross-border people flows who handle poppies on both side of the border.
On the other hand, it could also be introduced; the American strategy does indeed seem to quietly revolve around sitting on the poppy growers in Helmland, per Staretgy Page's "This Is The Plan". On the contra side, George W. Bush reportedly considered using pleospora in Afghanistan, but firmly rejected it. It's possible that Obama has decided to use biological warfare, I suppose, and the recent outbreak reflects that.
But how would we know?
That's the thing with biological attacks. Unless you're dealing with clearly unnatural mutations, like chimera viruses, you can't be sure it isn't natural. It doesn't take many people to implement. And even if it clearly isn't natural, you can't usually tell where it's from with any certainty.
Something to contemplate, as we face a religion for whom suicide-murder attacks on civilians have become, among many, the highest moral example. The pushback created in places like Iraq, where they were forced to engage and endanger their own co-religionists on a regular basis, has blunted that thirst - but not removed it. The Islamic civil war for the religion's soul continues. The winning side is very much in doubt. And the clock of falling technology curves still ticks....