This question was raised last Sunday in a NYT Magazine article by a former US senior counter-narcotics representative in Afghanistan. The facts presented:
1) Hamid Karzai is actively protecting poppy growers in Pashtun areas against effective eradication programs.
2) Farmers with other crop options are switching into more lucrative poppy cultivation; this is not a case of subsistence farmers with no crop alternatives.
3) The US military, the civilian Pentagon, and the Bush National Security Council have effectively opposed effective eradication and disincentive programs — as recently as 2007 (so this cannot be blamed on Donald Rumsfeld).
4) Poppy money is filling the coffers of the Taliban.
Obviously some nuance is required to not needlessly undercut Karzai, but the account makes clear that there was never a high level calculation of what an optimum strategy might be.