I'm saddened the horses had to be put down here, but I can't fault the police.
It was 2 am. I don't know any zoo or animal control that has staff to get a tranquilizer sufficient to put out a horse available at 2 am.
It sucks, but every other option has risks. The could not allow them to keep running around until they caused a major accident. (As for other options, I don't even know if the police would have been able to shut the road down quickly enough, assuming they'd have to shut both sides of the road down and cannot predict where the horses might roam.)
Would have been nice if someone could have alerted the horses' owner as well, but I don't know that she would have gotten out there quickly enough, either. Further, it was 2am, and I don't know if the police would have had enough officers to spare with two on the road trying to prevent accidents and yet another one looking for the horse owner.