Simply being the most dangerous low cost competitor against oils and chemical companies.
Excerpts:
http://www.unitedearth.com.au/thecreekblog/2011/06/marijuana-peakoil/
"Henry Ford’s first Model-T was built to run on hemp gasoline and the car itself was constructed from hemp!"
http://en.wikipedia.org/wiki/Medical_cannabis
"The Cannabis plant has a long history of use as medicine, with historical evidence dating back to 2737 BCE"