Question: Why shouldn’t behavior that clearly interferes with reason be made illegal?
Starting at the base of Objectivism with the premise that man needs reason in order to survive, why shouldn't any activity which objectively interferes with that ability be made illegal? Doing cocaine or heroin impairs the capability of people to reason. Drinking alcohol to excess also impairs reason. Why then should people be allowed to purchase and use these drugs? In a nutshell, I understand why drugs are immoral under an Objectivist philosophy, but I don't understand why anything which can objectively be shown to be immoral shouldn't be made illegal.
Answer: To engage in rational philosophical discourse, one must understand and bear in mind the differences between the subjects of ethics and politics. The purpose of ethics, from the Objectivist perspective, is to identify the values and virtues—i.e., the moral choices—that contribute to one’s survival as a rational human being. The purpose of politics is to discover the methods of organizing society that accord with those ethical conclusions.
In the Objectivist view, ethical behavior must be self-determined and self-driven. Therefore, the right political system is one that makes individual moral choice possible by banning the initiation of force and organizing its retaliatory use according to objective laws. As John Stuart Mill observes in On Liberty: "The only purpose for which power can be rightfully exercised over any member of a civilised community against his will is to prevent harm to others. His own good, either physical or moral, is not a sufficient warrant" (On Liberty, Ch. 1).
As separate, distinct beings, we can certainly be self-destructive or immoral without using force against others. As examples, you mentioned drug use, heavy drinking, and prostitution. Others would include suicide, excessive smoking or eating, listening to loud music (as long as you don’t bother the neighbors), and spending too much time in the sun without sunscreen. But while all these behaviors are ill advised or downright immoral, Objectivists hold that our actions are a matter of social concern only when we use force against others. Why?
Government can uphold laws and regulations only through the application of physical force. Whether it is arresting murderous criminals or collecting income taxes, it does so at the point of a gun. Objectivists are very concerned about the nature of force. It negates all possibility of choice for those subjected to it, and can enslave or destroy reasoning minds. The appropriate question, then, is: which actions are so heinous that a state should use force to prevent them?
Unlike most other political ideologies, Objectivism sets a high standard for the application of force. In particular, it holds that the only moral use of force is retaliatory, as a response to the initiation of force by some wrongdoer (or the plain immediate threat of such). So it is a contradiction in terms to speak of a "victimless crime." A crime, by its very definition, is perpetrated against someone.
There is also an unstated (and false) premise in your argument. You correctly state that man needs reason in order to survive, and that certain activities destroy or contradict our capacity to reason. Therefore, you ask, shouldn’t these activities be legally prohibited? But this presumes that government’s task is to ensure the survival of each of its citizens. This is not the case. The state’s proper role is not to exercise some allegedly benevolent ownership of your life, but rather to ensure that each individual citizen has the freedom—that is, freedom from the initiation of force—that he needs to pursue his own life and happiness.
Ayn Rand was careful to draw a distinction between the fields of ethics and politics. There is a marked difference between saying, "You should not do X," and "Stop doing X, or I will injure or kill you." While Rand believed that morality is objective and that it is possible to reach rational judgments about others’ behavior, she also contended that one must make moral choices and actions through the use of one’s own, individual reason. Prohibitionism—the enforcement of some collective or communal morality—would prevent citizens from exercising objective, individual moral choice.