To cover the original point - I have fostered quite a few cats who were at the vets to be pts, luckily my vat always seemed to manage to talk them round into releasing them into rescue care - although the last one was incredibly lucky, as I had just agreed for my last free space to go to a stray, so had to do a lot of juggling to save her, but I know that if my vets cant' find a rescue space, they have to honour the owners wishes.
I dont like the asthmatic reason, because most people do use it as an excue (I also have asthma), but I do appreciate that for some people it can be incredibly serious, people can die from asthma attacks.