It's probably the fact that one negative outcome (induced miscarriage) requires an action, while the other is passive (eventual death of babies / risk to mother). They *feel* more responsibility for the action than the inaction. If they'd done a selective reduction, they'd probably feel more guilt having committed the horrible sin of murdering the unborn. If the babies die through their inaction, on the other hand, it's so easy for them to say "out of our hands - after all, they *could* have lived!"