Cribbed from Jeffrey Alan Miron

1. Subsidizing alternative crops does not materially reduce production of the banned crop. Governments rarely provide sufficiently large subsidies, so most farmers keep growing the banned crop. Even with sufficient subsidies many farmers take the subsidies but then grow the banned crop anyway since enforcement is weak.

2. Crop bans breed corruption.

3. Crop bans generate easy income for precisely the groups the U.S. opposes (the Taliban in Afghanistan, the FARC in Colombia). These groups sell protection services to farmers and drug traffickers. Absent the bans, groups like the Taliban and the FARC would be much poorer.