Feeding a live kitten should be illegal, feeding a pre-killed kitten should be legal. However, because kittens and puppies are seen as cute and cuddly and a 'normal' pet, the majority of people will see feeding it live or dead to a non-cuddly animal (ie snake) as inhumane. Yet, a good portion of these same people, would dump unwanted kittens on the nearest animal shelter or worse, on the street, because they are too cheap to pay the $100 to neuter/spay them.
Shelters are full of unwanted pets almost all the time and though they do put alot of effort in to finding good homes for them, many just can't afford to keep alot of unwanted animals in their shelters for more than a couple weeks. So, I don't see why it would be wrong for them to sell/give away the dead unwanted pets to be used as a food source for natural predators, like snakes. It's really general public opinion, most of it there without real thought (it's a kitten, a cute cuddly kitten and that mean man is feeding it to a snake, how cruel!)
Though I love cats as well and do find it unnecessary to feed kittens or puppies to snakes, especially when there are other food sources available for them. (chicks, rabbits, guinea pigs, etc.) I certainly wouldn't protest against feeding unwanted kittens/puppies or adult cats/dogs to large snakes, providing it is done in a humane manner. Then again, snakes are very efficient killers and it doesn't take long for them to kill their prey. I rather see people put more effort into stopping dog fighting than a reptile owner recycling unwanted pets as food for repties.