Here is their stance...
Basically, if it's a stray, it's ok, but you shouldn't buy from pet shops (because of puppymills) or let your pets breed. It seems like their idea is that if no more puppies or kitties are bred, eventually there won't be any more, so none of them will suffer.
I've also heard that once they rescue animals from shelters/puppymills/fur factories, they kill them anyway. It really seems like they're more for the extinction of animals than anything.