Maybe it’s just me, but I can’t see what’s wrong with killing pests? After all, that’s what strays picked up by animal control are a lot of the time, introduced species killing native wildlife. Sounds like a good idea to me, those shelters are crowded up.
But I do not approve of the whole killing pets that are microchipped.
Because we can’t kill an animal if it’s cute, that would be wrong! Look at its big floppy ears!
I don’t think anyone would buy enough dogs and cats. 
Seriously, cars are worth something. Dogs and cats really aren’t. Now something that you can eat, milk, or wear like livestock could be impounded longer if re-sale would pay for their board. 