The SPCA has historically hired animal disposers as employees, not animal lovers or even animal experts. The result has been SPCA employees who are efficient at processing the goods that stream through its doors every day by the hundreds. The SPCA does not need to ever say, We are full and cannot do the right thing by your animal, so we are not going to take your dumped pet, because it can kill them or sell them to another irresponsible person (the "irresponsible people" that the SPCA so cleverly blames for all the pet dumping but who are its bread and butter, the free suppliers of all its product).
Does a dumped dog have separation anxiety, probably from being left alone too much? No problem, we'll sell it to someone who will isolate it again, and we'll keep selling this dog until we can't find anyone to buy it and then we'll kill it. This is how the SPCA "moves product", at least the product that does not die of SPCA pest hole diseases and is still alive to be sold.