If the home owners do it, then SPCA wouldn't have to.

You cannot care for all of your pet's offsprings. Eventually you will need to give them away, and hopefully to good owners.
But you'll never know. Some of them might ended up as a stray. If they managed to survived, then they'll breed more. Who is to give a good home to them?

You are equating a human and a dog, two distinct beings with very very different mental capacities.

Perhaps we should think of it this way, that it is a bit on the cruel side to neuter/spay your pets but it is a necessery evil that we do so that our society can function smoothly (stray animals are always a hazard to us, road accidents, diseases, etc).

By neutaring/spaying them, we also remove from them a few potential health hazards and also the mating urge that most of us will not let them engaged to.

It is childish to not acknowledge all these issues that affect both us and our pets while standing on a misguided moral high ground.

Because ultimately, we know that all these breeds of pets that we have are actually animal monstrosities that we human selectively breed out.
They are not natural, and because of our tempering, they are now susceptible to various health problems that is specific to their breeds.

If neutering/spaying is cruel and we should not do it, then we should not have all these different breeds of a same animals in the first place. Let them breed amongst themselves without our interference.
Heck, we should not keep pets at home at all. We should let them run wild and true to their nature.