When you buy a car battery or tires you pay a fee to help offset the environmental costs. My clumsy analogy is that un-spayed / neutered animals are an environmental side effect to domestic pets, and a robust (and easily funded) spay and neuter program is both humane and economical. I am the happy owner of a succession of shelter cats. Maine's animal shelter system has done a great deal to manage the problem of unwanted pets - our very low rate of euthanasia is almost certainly the result of the Help Fix Me program.
I am very pleased to report that as of yesterday, the Agriculture Committee voted unanimously to drop this proposal from the budget. It's not dead yet, but it's been dealt, I am happy to say, a severe blow.