I reckon the cop who got bitten thought - right, I'm going to get you you *******, jumped in his car and flattened the mutt at 90mph. Once the red mist had receeded, they cooked up the justification story.
I have no evidence for this wild speculation...
Come on - it was 3am. The road would've been empty. This action would've caused expensive damage to the police vehicle too. It just seems like a bizarre decision.