The introduction of two poisons into society does not necessitate the introduction of a third, fourth and fifth.
I really doubt cannabis is a "poison" in any comparable sense as alcohol or nicotine. In any case, suppose that it is: suppressing the introduction of new poisons into society doesn't come for free. It requires regulation, enforcement of drug policies,
increasing the number of criminal offenders shuffling through courts and prison systems.
At the same time, we need to consider the actual benefits of legalizing new "poisons". There is a market for drugs, about
$32 billion in untaxed revenue circulating through the US. There is a strong argument that legalization can move drug money out of the hands of shady criminals and into more reputable business owners.
The question isn't so simple. We really have to consider whether enforcing drug laws is a net positive return on investment.
I don't believe there is a credible case that criminalizing cannabis has any advantages at all. In fact, a growing consensus of experts around the globe suggests the
War on Drugs is a failure, that it has failed to reduce drug usage, the harm of drugs is grossly exaggerated, and the tremendous cost of fighting the war is a net negative.
I personally believe legalization of cannabis would have a hugely positive impact on the US. Economic impact includes recreational drug users, but the many many
uses of hemp, reduced burden to prosecute non-violent drug offenders, etc. There are huge advantages to decriminalizing or legalizing cannabis, certainly many more advantages than prohibition -- those advantages justify legalizing cannabis on that basis alone.