I think most of these miles are to move and house people not animals.
PETA wants free airline tickets, hotels and rental cars.
THe Golden Ret site puts it best. The miles buy a ticket for a person, who will then fly a dog. That is very inefficient, compared to van or trailer with 5-10 dogs.
In most cases, there are plenty of abandon dogs of a particular breed in a particular area to keep the rescue league overstocked. I realize Greyhounds are unique, and I participate in the Brittany Society Rescue, which moves dogs from KS and MO to the east coast.