Another HUGE difference between most animals that are put down and Terri is that the animals clearly are dying, Terri WAS NOT. Barbaro lived for nine months after the Preakness because his owners had unlimited financial resources and had access to the most advanced veterinary specialists in the world. Had he had this same initial injury a decade or two ago, he would have been put down on the track.
This is exactly the point - Terri was not dying.