Don't blame the SPCA, blame the individual who made the decision to kill their dog. I don't know how many dogs are at your SPCA, but if it has a high-population of animals, with new ones coming in alot, then they might have seen nothing better to do than put down the adult mutt who had been there for a month, rather than the cute puppies who had just arrived with their mom.
It normally is not a group decision on put an animal down, it's up to a few people who goes, and who stays. The last person in charge of our SPCA was putting quite a few adoptable animals down, and still thinking they were doing good, thankfully enough volunteers got him fired, and now there is a sensible person there.