The same way people can defend the legality of the worst drug on the face of the earth by far: alcohol.
Alcohol is an addictive, mind altering, body destroying substance, which causes more death, violence, and suffering than all illegal drugs combined.
It's the height of hypocrisy to legalize alcohol while making weed and other substances illegal.
Putting people in prison for posessing the wrong substance is not justice. People are responsible for their actions, whether influenced by illegal or legal drugs, and if they infringe on someone else's rights during the course of pursuing their bad habits, only then should criminality apply.
The powers granted to government by adherents to the Fascist Drug War are overtly Tyrannical, and have nothing whatsoever to do with Justice. This is why, in "the Land of the Free", there are more non-criminal citizens in prison than practically anywhere else in the world.
The Drug War is a Tyrannical bad joke which does nobody any good.