It has ever been thus, guerrilla forces get their arms from the forces they are fighting against. Buying, bartering for drugs, and stealing are the logistical basics for guerrilla war. There will always be underpaid colonial troops willing to sell their rifles to feed their children. As for the scared, sleepless occupation grunts, the allure of raw opium and the numb oblivion it provides is a welcome relief in exchange for a little ammo.
I am not surprised that the Pentagon is uninterested in tracing the source of the Taliban's weapons. They desperately don't want to know - it's Don't Ask, Don't Tell.