I think you're correct. Normally they'd have just shot it. However it would be hard to claim that a 14 YO chihuahua was a danger to them without seriously affecting their "big man" standing in the department. Better to just leave it on the side of the road and let nature take its course.
I was thinking of the dog's presence there might CAUSE an accident.
*SOME people might have tried to stop and rescue the dog.
*Some might have slowed down enough to cause an accident.
Lots of "what ifs" but they are all moot since doggie is dead.
If the cops in Houston have the camera on them all the time, like some PDs have, then the cop who did that is known. I doubt he would get into trouble anyway. Cops often turn into some very hard, cold hombres.