My local RSPCA has this policy (CP have rehomed them, or if they have other issues, allowed them to live their life out), I did question her, and her reasoning was along the lines of it was best for the cat, as while they may appear ill, they may not be and it is to help prevent them suffering and she is the manager, so her rules go. The rescue's local RSPCA dont, so I have to say I would support them over my own. The other aspect to it is that it sounds like the snap test (and I know that is all my local RSPCA use), and there is a 7% chance of it being wrong, so potentially 7 out of every 100 will lose their life unnecessarily, they can't be following it up if it is done on the day they are taken in.