This same topic was raised on Iherp back in October, and some good points were made.
http://www.iherp.com/Public/Blog/Detail.aspx?uid=88434

Post #16
Actually, the natural choice of food for a lot of my snakes would be more along the bird and lizard variety, they have to be taught to accept a rodent diet.

I see rabbits and guinea pigs bagged up and offered for snake food, those are definitely pet creatures in most people's eyes, especially the guinea pigs. Puppies? Sure, ok, add them to the frozen food menu as well for the sake of discussion. May as well put ferrets on there too since I'm going there. My point is that all of those animals that we feed our snakes have value as companion animals, they're all domestic. At one point in my life, all I had for companion animals were my rats and I know first hand that they have amazing personalities and make great companions. I don't see bags of frozen rats as the same thing as the neat minds I knew way back when, I can dissociate and just think of them as food items. I can't imagine feeding kittens or ferrets because they're both obigate carnivores and I doubt the nutritional value would be the same as it would for a herbivore or omnivore. Besides, what snake in its right mind would eat a stinky ferret...lol. Puppies would probably make a very nutritious meal, perhaps even a bit too fattening. Kittens I may be able to disassociate with, though I've known and owned (gasp!) some pretty cool cats in my time (clearly the minority!) so its hard to say. Puppies, I highly doubt I could cross that line given my relationship with the dog species as a whole. Yea, I don't think I could go there. I don't agree with scamming stuff off of CL to feed to snakes because its just wrong on so many levels, but were there puppies and kittens on the frozen menu at RP alongside the bunnies and guinea pigs, I'm not sure I'd judge someone else for buying them to feed to their snakes.