Is there a genuine case to be made for outlawing marijuana given the fact that alcohol and tobacco are legal? In other words, is there a way to make a distinction between marijuana on the one hand and alcohol/tobacco on the other hand such that it will appear legally justifiable to outlaw the one and not the other?
Sure, such a case can be made. Let's suppose we view all three substances as detrimental to society. We can still argue that practically speaking once a substance has the very long history of legalization and has become entrenched into society the way alcohol and tobacco are (and marijuana is not) it is very difficult to simply outlaw it. Yet, we might take steps to discourage their use... for example, the USA has been quite successful in reducing the smoking rate through education campaigns and by adding excessive taxes on tobacco. All of this is perfectly consistent with saying we should not legitimize a third addictive detrimental substance like marijuana into society by legalizing it.