That doesn't make logical sense to me. Just because the one being dragged is unconscious, that doesn't make him/her impossible to attack. I would rule that the dragger and the drag-ee become a single unit for OA purposes, and the attacker may make an Opportunity Attack against either of them, but not both. And if the dragger has some kind of parry ability, he/she could use it to defend the unconscious drag-ee.
It's not as though "being dragged" is any more provoking than "just lying there unconscious".
Voluntary movement provokes. Involuntary movement doesn't.