The biggest problems I see is that
1) the horse isn't tied so he has the freedom to move around the entire back end of the truck.
2)the "walls" of the enclosure are very short and that horse could have easily jumped out if he had gotten spooked while they were driving along.
3) the horse has the ability to stick his head out around the white thing in front of him. I've had windshields broken by rocks and/or debris thrown up by a passing vehicle so I can only imagine the serious damage that could do to a horse...especially if it hit him in the face.
I have zero problem with people hauling horses in the back of a truck like that...so long as the enclosure is appropriate and safe.