I wondered this too, as I have wondered what I'd have done in this situation. If a lone male officer in an unmarked car wanted to handcuff me, when I hadn't committed a violent crime, I would think I'd have resisted that - but surely at this stage he is so set on his pursuit, that he would have used force to get her into the car?
(btw I am not victim blaming in saying I would have tried to resist - I am always very dubious/untrusting nature, while Sarah quite rightly trusted an official police officer's word. But just speculating on how things would have gone down if she had tried to resist - I think he'd have used force anyway).