“The sort of thing that would get guys arrested now was very common back then,” said Michael Walker, who made a study of the Los Angeles sex-and-drugs scene for his 2006 book, “Laurel Canyon: The Inside Story of Rock-and-Roll’s Legendary Neighborhood.”

Mr. Polanski was treated by the authorities, including Judge Laurence J. Rittenband, not so much as a sexual assailant but as someone in the mold of Isaac Davis, Mr. Allen’s character from the movie “Manhattan”: that is, as a normally responsible person who had shown terrible judgment by having sex with a very young, but sophisticated, girl.

Contemporary reports in The New York Times and elsewhere captured a whiff of that attitude. In one article by The Associated Press, published in The Times on Sept. 20, 1977, Judge Rittenband scolded Mr. Polanski for taking advantage of his victim even as he was “noting the teenage girl ‘looks older than her years’ and was sexually experienced.”