When I first read this post, my risk manager side said "This seems to be outside the scope of his employment, I wonder why the county covered the civil liability".
I then read the article and saw that the officer was in uniform and driving his police cruiser, so there could be a reasonable presumption that he could be considered on-duty, and the employer is vicariously liable for his actions. But more importantly, by claiming he was 'on-duty', so to speak, the county's legal liability was limited by governmental immunity to $ 200,000 per claim. With the one claim of the child and the claim of the parents, it adds up to the $ 400,000 paid by the county. If they weren't shielded by the governmental immunity, the county would have paid much, much more. So from the county's perspective, it was a good decision to claim that they were responsible for the officer, even if he was off-duty and moonlighting. It kept down their costs, thus saving the taxpayers money.