I'm from Canada Bossroo, so no clue about the US shelters. It was pretty standard in the Canadian shelters as they were trying to limit more unwanted puppies down the road. I see the vet/rehoming fees in the US are much higher than in Canada also...vets that did the spay and neuter program in my area when I lived in Canada, did it as volunteers.
My guess as to why they do these programs is to try bring the number of unwanted pets to a lower number...most shelters are on overload.