You are missing my point. I think the Afghan farmers should be growing opium poppies (in a controlled manner). I don't think it's throwing gas on the flame, or at least not on one that wasn't already fully ablaze. It's a needed commodity and for now a fundamental and irreplaceable crop of Afghanistan's agrarian economy. The US and other NATO proponents are trying to control or guide the legitimacy of it, not profiteer. It's not so much faith in foreign policy as it is realizing that it's not a simple answer of eradicating or cultivating.
Did you know that the Taliban only banned opium poppy cultivation in 2001, and prior to that Afghanistan was still the top producer of opium poppies? It's not like Afghan farmers went from twiddling their thumbs to dealing smack as soon as the US arrived. The Taliban ban on opium poppies devastated the Afghan agrarian community, who already lived in one of the poorest and most drought-ridden countries in the world. Before the ban, the Taliban sanctioned opium cultivation as permissible because its derivatives were ostensibly consumed by non-muslims. But there is also strong suspicion that the Taliban opium ban was a ploy to drive up the value of the opium they had stockpiled. After the US invasion, the Taliban ramped opium production back up to support the insurgency effort. The UN estimate is that the Taliban made $300M on opium in 2006.
So since there is already evidence of what an outright ban on poppy cultivation does to the Afghan farmers, it's understandable why the US and NATO are cautiously negotiating the opium poppy industry in Afghanistan.
For what it's worth, I have a friend running an NGO in Kabul which argued against the US's initial inclination to just eradicate poppy crops. I get a bit of an inside scoop.
On Colombia: You really don't understand Plan Colombia (the actual plan - not the movie, which I haven't seen). The usual criticism of Plan Colombia is that the Colombian military destroyed legitimate crops while fumigating coca crops, not that they were destroying legitimate crops to force coca cultivation. Plan Colombia was a US-supported anti-cocaine campaign.
I have no doubt that there was Colombian corruption countering the positive objective of Plan Colombia, and there were adverse implications resulting in displaced farmers, and the effect were not immediately realized in the US due to cocaine's lead time to market,a and there was this underlying and sometimes conflicting "real" purpose of incapacitating guerrilla forces who fought against the Col. gov't and were funded by cocaine trade. But Plan Colombia itself was not, as you suggest, a means to increase coca production and get it on US streets. There is no indication that the US had any intent to increase cocaine volume into the US or jack prices from reduced supply. Price increases would actually be looked foreword to by the US because it is what they use as an indicator of a slowing importation rate.