Marijuana is much less dangerous than heroin because it is impossible to fatally overdose on it. Although there is such a thing as a marijuana overdose, symptoms of which can include nausea, vomiting, and panic attacks, it only lasts for a short while and wears off without any long-lasting negative effects, unlike a heroin overdose, which can often be fatal.
I don't think that anyone who knows about drugs thinks that marijuana is as dangerous as heroin, but that does not mean that it is safe. Marijuana has many harmful effects, especially when it is taken by teens and young people. Just because it is not as deadly as heroin does not mean it should be legalized.
Herion is a very deadly drug that has caused people to overdose the first time they do it. Marijuana on the other hand, is relatively safe. No one has ever died from an overdose. There is also not the same level of addiction to marijuana as there is to heroin.
Marijuana is a natural plant that grows out of the earth. Heroin is heavily processed. THC has no physical or chemical addictive qualities. Heroin is highly addictive. Marijuana cannot lead to an overdose. Heroin can kill quickly and indiscriminately. Marijuana is inhaled or injested. Heroin must be snorted through the nose or injected with a needle. To compare these would be like comparing a strong cup of coffee to a handful of what's in every medicine cabinet across the country.