I think it depends on the reason. I try to remember that adoption agencies get some really out their people who on a whim filled out an application. When we sell a horse we ask for a vet reference. This is in part to make sure they have a vet and also because the number of "hoarder" type owners we have encountered is high. It also scares a portion of people off.
I also think that most of the time these adoption agencies are running on few donations, lots of volunteer sweat, blood and tears. Which means that for them adopting a horse out can be really hard. So if they know that the horse is well cared for they feel better about it. They also sometimes take in horses in less than ideal condition (skinny, wormy, spoiled) and they hate to see that horse return to a condition that is less than or equal to as bad as the one they took the horse out of.
I also try to remember that sometimes rescue (of any type) is less about saving the animal and more about the people saving themselves. As an example, I heard of a woman who ran a newfoundland dog rescue. She had been raised by a abusive/alcoholic father. She had a New York City basement apartment and had 7 newfies, one of which had paralysis of the back legs. For her, saving those dogs was less about the dogs and more about her saving herself. So, I try not to judge an rescues to harsh. If they are able to care for the animals in their possession well than that's what matters.