You're a very nice person. But you just don't understand the magnitude of the problem! Those $20.00 a month don't even cover 1/10th of the pets that the ASPCA has to deal with. So they are faced with ignoring the pets running loose on the streets, or to take in and euthanaze the less "adoptable" pets in a more "human" fashion.
I don't like it. . .it breaks my heart, but I rather they be euthanized than end up in an experimental lab, hooked up to tubes, or with chemicals poored into their eyes so that we, humans, can use make up with no pain. I rather they be euthanized than used as "baits" for dog fighting rings, or be left to starve to death chained to a tree in the forest.
This is, unfortunately, a very parallel situation to the many unwanted pregnancies in humans.
Unfortunately, this is not "Eden." We live "EAST of Eden." And this world is very far to be perfect. What we can do is try to prevent those atrocities through prevention.
Is your dog or cat neutered or spayed? Are all your friends' dogs and cats neutered or spayed? When you are ready to adopt a new pet, do you pick one from the local SPCA or from a rescue center? Do you pick a "less lovable pet" (older, not attractive, with some behavioral issues), or do you pick a cute puppy from a puppy mill?
Those questions can make a (small) difference. You just can't solve the whole issue at this time, when spaying and neutering are not an obligation, without resorting to euthanesia.