The belief that heroin or cocaine cause addiction has been seriously challenged for a while, you should read "The Myth of Drug-Induced Addiction", the speech to Senate of Canada by Bruce K. Alexander of rat park fame.
While rat park is fascinating, it's not the only part of the picture by a long way, and his views aren't exactly mainstream even amongst controversial and outspoken drug scientists like Prof Nutt.
I would be very hesitant to call addiction a myth when we can measure it, and measure the relative addictivity of various substances.
Bruce K Alexander has some interesting views and some unteresting data but AFAICT has failed to really prove his case or overthrow the current models.
http://www.brucekalexander.com/articles-speeches/demon-drug-...