the cops suspected he was drunk. they did not know for sure.
as for releasing the dog on the perp, that was the right call. dogs are faster than humans. this was an unusual case. 99 out of 100 times, the dogs are fine and they subdue the perp. the only reason this went bad was because it was on a bridge.
Humans have judgment, the dog does not. The dog is dead because humans sent it in rather than go themselves. A human would have let go rather than be dragged over the side. The dog was trained not to let go except on command. Result is a dead dog.
Dogs are effective in apprehension due to fear factor. A solo dog can not stand up to a trained human. Its one of the reasons you see the handler stay close to the dogs these days. Years ago doctrine was to send the dog in alone. PDs lost a lot of dogs that way.
Not saying the perp did right, but the most likely answer is that the handler effectively sent the dog to its death needlessly.