Thats a big part of the problem, there are so many parents out there that view pets as disposable items, who have irresponsible attitutes towards pets and then they pass these lack of values on to their kids. I grew up in a family where we committed to the life of the pet. I now have kids, as well as a zoo (3 dogs, 1 cat, 2 rabbits, 1 cockatiel, 1 snake, 1 skink and several turtles) and I frequently have other parents that try to pawn off their unwanted animals on me and it really makes me sad and I'm thinking "I can commit to my animals for their lifespan, why can't you?"
So often its just their tired of taking care of it, or they expected the child to accept full responsibility for it, and the child doesn't so the parents want to get rid of it.
We ask our kids to help with the pets, but ultimately it is ours (the adults) responsibility). I would never make committing to a pet conditional on how much the child is willing to take care of it. My daughter is good about feeding the dogs most of the time, but sometimes I nag, and it doesn't get done, so I feed them. She still does alot of other things for them, she bathes them, grooms them and walks them. So I see it as a shared responsibility of a family.