Honestly, I wouldn't want a former pet such as a dog or cat as food for my snakes. Morals aside, you never know just how many medications those animals have been on in their life time. Some of those medications are still in their system and makes them unsafe for consumption, even if they weren't chemically put down.
Its the same with horses. I would NEVER eat horse meat unless it was from a horse that was raised for slaughter. Its not just that I love horses, but I know what goes into a horse on regular basis. Horse people are crazy when it comes to going above and beyond to make their animal comfortable. There are many drugs and pesticides given regularly to any horse. Not to mention the many meds and shots they are on, for various problems and issues.
Some of those do not break down quickly. And trust me, you don't want those in your system..
While I agree it would be better use then thrown in the trash, that probably would be a slippery slope. Many people would not believe those animals were killed humanely. There would be an uproar, because dogs and cats are viewed as highly intelligent and affectionate companion animals, at least in our country. As is, the people collecting free kittens and puppies to feed them to their snake, are throwing a bad light on the rest of us reptile keepers. Esp. when they go around promising to give those animals good loving homes.