Pre-internet, pre-AAS, pre-Donna Liberson and a thousand other watchers and voices, the SPCA just killed all the dogs that might cause it to be sued. Killing poor product was the SPCA's way of doing the pet disposal business. It called all the killing 'animal welfare' and for many decades it got away with this.
Almost the only people who knew it was not animal welfare were the real animal welfarists who instinctively understood that intaking all product and culling out the unsellable, is morally indefensible. But from a business perspective it makes perfect sense as it provided hundreds of jobs for many decades, some of them very highly paid.
The women who for decades did the work that the public gave the SPCA many millions a year to do, who raised the money with bake sales and garage sales, who become impoverished because they could not bear to betray the animals they knew were suffering, by killing them or denying them medical care, did not need any training in ethics; they acted out of innate morality.
The question must be asked over and over: can the kind of people who run an "animal welfare" society that kills helpless animals ever innately understand the ethics of animal welfare?