Whether or not the SPCA has a euthanasia policy and whether it is used accordingly is of little significance. Animals are supposed to move through its clogged system at a steady pace and if for whatever reason(s) they don't, death becomes a matter of inevitability. In the past I know from being told by SPCA staff, that the policy has been used as grounds to justify the killing.
And as long as society regards animals as expendable property, and the sanctity of life is ignored, there is little hope of change.