I have trouble believing that. Any doctor who advised a patient to do that, or who performed an abortion knowing the patient was doing it based on the belief that her positive triple screen meant she was carrying a Down Syndrome fetus, would be asking for a colossal and losing malpractice suit, followed by career-ending uninsurability, if the patient actually acted on that advice. Just because a doctor tells a patient after a positive triple screen that abortion is an option, does not mean the doctor is suggesting that no further testing needs to be done to make that decision.
The triple screen is like a mammogram, a first-pass screening method. If the mammogram finds a small lump, it makes sense to remind the woman that IF the lump turns out to be cancerous, mastectomy and chemotherapy are options that will likely take care of the problem -- not pleasant options, but options. But no doctor would recommend that the patient go ahead and have a mastectomy, without first doing a biopsy to find out whether the lump actually is cancerous.
I repeat, it happened to me. It was in May, 1989. (By the way, my almost-17-year-old son is just fine, thank you.) If you think there aren't doctors out there that offer abortion on the basis of little or no indication, you're quite mistaken.