I am sure that sometimes you are correct in saying that dogs and cats can get used to each other. However, it is part of our rehoming agreement that original owners do have a say in where their cats are rehomed to. This is because in many cases they are rehomed not because of neglect or cruelty but simply because of changes in their circumstances and they understandably are concerned about the cats future homes. In a case such as Tink where they have been in foster, then the foster mum or dad takes the role of previous owner. In Tink's case she is a very dominant cat who loves nothing more than to put older, weaker or less active cats in their places, quickly!!!
She is a lovely girl, very friendly to humans, but will brooch no nonsense from other animals. So, in this case she needs a home with no small children for her to bully and no animals for her to dominate. Dogs as you say can suffer from cats claws, and we feel that it is our responsibility to protect the innocent, i.e., dogs already living in the home. Of course if the cat for rehoming is used to living with dogs, we would then place them in homes which have dogs already as part of the new family.
We do not have the luxury of a centralised rehoming centre and our fosterers are volunteers, who may or may no be dog owners, and of course that dictates whether we have the ability to assess the cats tolerance of dogs.