3. It has been historically linked with narcotics, such as heroin.
Early antidrug laws were written to regulate narcotics--opium and its derivatives, such as heroin and morphine. Marijuana, though not a narcotic, was described as such--along with cocaine.
The association stuck, and there is now a vast gulf in the American consciousness between "normal" recreational drugs, such as alcohol, caffeine, and nicotine, and "abnormal" recreational drugs, such as heroin, cocaine, and methamphetamine. Marijuana is generally associated with the latter category, which is why it can be convincingly portrayed as a "gateway drug.".