Barbara Walters kept asking Patty Hearst, on last night's edition of the ABC news magazine "20/20," why she had done this and why she had done that. But the question at the heart of the matter seemed to be, why did Hearst consent to a grilling by Barbara Walters about a sordid past anyone else would be eager to forget?

The answer is so simple it hurts. There was a book to be plugged -- a holy rite in television that can cure the camera-shyness of anybody from G. Gordon Liddy to, were he still alive, the Elephant Man. Hearst's book, "Every Secret Thing," has just been published by Doubleday. A half-page newspaper ad yesterday urged readers and potential book-buyers to catch the Patty and Barbara show on "20/20."

Wearing her grimly serious and moderately reverential face, Walters wasted no time in getting to the seamy side of the street. She twice mentioned, as preface to the interview, that Hearst was "raped" by members of the Symbionese Liberation Army, the radical kook group that kidnaped her in 1974 and which, it appeared, she later willingly joined for such escapades as robbing banks.

Hearst said that she spent more than 50 days in a closet -- "That whole closet experience," she called it -- and that her "torture" included being observed while she used the bathroom. But Walters persisted in her inquiry about sex in the SLA: "Whatever they wanted, as many times as they wanted, that was it?" asked our inquiring camera person.

A short time later: "And the sex? Anyone who wanted it just demanded or took it or what?" When Hearst broke the mood of furtive solemnity by giggling about the way the "comrades" held meetings over sexual privileges, Walters all but See HEARST, C8, Col.1 HEARST, From C1 scolded her back into sobriety. Later Hearst more or less volunteered, of her relationship with one SLA member, "He sort of had me whenever he wanted, which was not all that often. . . ."

Walters showed up at Hearst's "unpretentious" but heavily guarded home with video and audio tapes of those bygone SLA days and played them to refresh Hearst's memory; it was a little like a bizarre commercial for the greatest guerrilla hits of the '70s. Hearst looked befuddled and blank, her face often freezing into an expressionless gaze. She didn't have particularly cogent answers for Walters' questions about why she failed to escape even when given ample opportunity.

Asked why she didn't phone her parents while alone for days in a Las Vegas hotel room, Hearst shook her head, stared for a moment and said, "I just couldn't. It's terrible." Asked why she didn't flee the clutches of the SLA when not guarded, she said, "I couldn't, I just plain couldn't."

The report ran for about 32 minutes, roughly half the broadcast, and the first 21 minutes was aired without interruption by, or delay for, a commercial break, perhaps a ploy by ABC to snare and hold viewers in light of the fact that NBC's "Hill Street Blues" appears suddenly to have become a hit in the same time slot.

There was a certain eery allure to the interview, and to the shots of Hearst, her gun-toting cop husband, and their little six-month old daughter, Gillian, strolling near their home with a surly German shepherd as a bodyguard. But basically, the interview proved again that no news dates like tabloid news; it's like an old fad or a foolish fashion craze come back to haunt us. The whole case seemed as distant and irrelevant as the Lindbergh kidnaping. But then, people are still writing books about that one too.