No qualms vexed John Kaplan eight years ago when, as an assistant U.S. attorney in San Francisco, he put drug pushers behind bars. A nonsmoking teetotaler, he had little sympathy for drug users of any kind. Later he became a law professor at Stanford University, and the California legislature hired him to help revise the state's drug laws. Then a surprising thing happened: the legislature fired Kaplan and four other professors working on the project because, after three years of exhaustive research, they reluctantly concluded that marijuana should be legalized.
Now Kaplan, 41,...