Alcohol and tobacco are both legal, and they are far worse for public health than marijuana. In the U.S. roughly six people per day die from alcohol poisoning, and about 480,000 deaths per year are linked to smoking tobacco. There is no reason that while these are both legal, marijuana, which is nontoxic, should not be.
Just because one thing is less harmful than another does not mean it should automatically get the same treatment. If alcohol and tobacco are significantly worse than marijuana, maybe laws around them should also be examined. Additionally, reinforcing regulations of marijuana are much harder than with alcohol. For instance, while with alcohol a Blood Alcohol Limit can be implemented to stop people driving while intoxicated, no such parameters exist with marijuana despite it impairing users' driving abilities.
[P1] Alcohol and tobacco cause huge numbers of deaths each year. [P2] Marijuana is nontoxic. [P3] If alcohol and tobacco are going to be legal, there is no reason marijuana should not be.
Rejecting the premises
[Rejecting P3] Just because marijuana does not cause as many deaths directly as alcohol and tobacco does not mean it should be legal.