I have been told it is because cannabis is suspected to possibly have no medical use and it possibly has a high potential for abuse. There are people currently doing research to discover whether or not these facts are true.

Opossum facts: When angry or in fear opossums make a hissing sound or a grunting growl. When an opossum gets too scared it will play dead. The opossum will go into shock and faint, falling on its side while curling its lips upward to give the appearance of being dead. While playing dead they will secrete a foul smelling mucus from a gland near the tail, the repugnant odor will further enhance the illusion that they are dead and help to possibly prevent a predator from wanting to eating them. The playing dead state of shock is an instinctive involuntary response and not controlled by the opossum and can last anywhere from minutes to several hours.