Why is there still the rumor that tetrahydrocannabinol (cannabis, marijuana, hashish) is a relatively harmless drug?

From http://www.marijuanaaddiction.info

Marijuana addiction is characterized as compulsive, often uncontrollable marijuana craving, seeking, and use, even when the individual knows that marijuana use is not in his best interest. Marijuana is both emotionally and mentally addictive. Once an individual becomes addicted to marijuana it develops into part of who they believe themselves to be. Avoiding their friends who do not use, the addict will gravitate to others that do. Marijuana has psychoactive and physiological effects when consumed, usually by smoking or ingestion. The minimum amount of THC required to have a perceptible psychoactive effect is about 10 micrograms per kilogram of body weight. Scientists now know many facts about marijuana’s effect on the body and how delta-9-tetrahydrocannabinol (THC), the major active chemical in Marijuana, acts in the human brain. When marijuana is smoked, THC travels quickly through the body and into the brain where it unites with specific receptors on nerve cells. Areas of the brain with the most receptors affected by THC are parts of the brain that control pleasure, thought, memory, sensory, concentration, time perception, and coordination.

Marijuana use has been shown to be associated with several illnesses. Masijuana has been associated with lung cancer. Whilst some studies and tests have proven inconclusive, a recent study by the Canadian government found cannabis contained more toxic substances than tobacco smoke. It contained 20 times more ammonia, (a carcinogen) and five times more of hydrogen cyanide (which can cause heart disease) and of nitrogen oxides, (which can cause lung damage) than tobacco smoke.

More recently, the Dunedin Multidisciplinary Health and Development Study published research showing an increased risk of psychosis for marijuana users with a certain genetic predisposition, held by 25% of the population. In 2007, a study published in The Lancet and a poll of mental health experts showed that a growing number of medical health practitioners are convinced that marijuana use increases susceptibility to mental illness, accounting for 14% of United Kingdom psychosis cases. Cannabis use can also lead to anxiety and panic reactions. There is also evidence that some of the above effect can become permanent with heavy usage. Multiple studies have shown that chronic heavy marijuana smoking is connected with increased symptoms of chronic bronchitis, such as coughing, production of sputum, and wheezing.

My view on the subject? Yes, there are far more dangerous drugs like meth or crack. Yes, there are far more alcoholics than cannabis addicts. But does this make cannabis harmless? It’s possible to drink alcohol in moderation and lead a long and successful life. Is the same possible for marijuana use? I’m not sure. And why the rumor? Has it do to with the student movements of the late 60ies? The Woodstock festival glorification? Any thoughts?