Note that I’m not asking whether or not the church approves of drug use. I’m asking if they think it’s morally acceptable for harder drugs to be legalized.
The Catholic Church doesn't exactly anathematize libertarians because she doesn't anathematize anyone any more, but Pope Leo XIII, for one, has some good words to say about the spirit underlying the cause to legalize drugs; even if they're not used.
A position that a thing is OK to legalize, but not OK to do is entirely contrary to the meaning of law as understood by the Church. I think Pope Leo XIII would have lumped this paradoxical position as yet another facet of Americanism. This idea goes against the Categorical Imperative (which is not Catholic doctrine either) but works in this sense, if we can't all "do drugs", none of us should "do drugs".
Anyway, St. Thomas Aquinas summarizes the purpose of human law in a sentence:
the purpose of human law is to lead men to virtue, not suddenly, but gradually
He objects to the notions that human law can make people virtuous, repress evil or stop vices.
Legalizing drugs, makes it easier for more people to not be virtuous. Prohibiting drugs makes it harder for a few people to be virtuous (i.e. not participate in illicit trade of contraband).
I'm not saying the needs of the many outweigh the needs of the few. I'm not saying that I wanted my brother, for one, sacrificed at the altar of human law (I have no doubt he would still be alive if it weren't for California's drug laws). I'm not saying that the Church supports laws that perniciously target minorities. Pope Leo XIII also wrote that laws should be distributive. That they should be for everyone. A law that legalizes drugs is a law for existing and up and coming drug users, it's not a law for people who would prefer that drugs played no part in their lives.
The only thing that would be acceptable, is to forget laws about drugs and trust to people's good conduct. But since everything in America seems to need laws around everything, the options are to make concessions for use or prohibit. The only moral choice is to prohibit.
In "Whether human law should repress all vices?" (Summa Theologica I-II q. 96 a. 2 co.), St. Thomas Aquinas shows that human law should forbid the vices from which the majority can abstain:
human law is framed for a number of human beings, the majority of whom are not perfect in virtue. Wherefore human laws do not forbid all vices, from which the virtuous abstain, but only the more grievous vices, from which it is possible for the majority to abstain; and chiefly those that are to the hurt of others, without the prohibition of which human society could not be maintained: thus human law prohibits murder, theft and such like.
The majority can abstain from marijuana and cocaine.