Crazy Libs do this same thing to their pets. They claim their dog or cat is vegan and only feed them veggies. They are surprised when the fur starts falling out and ribs are visible. So they feed them more carrots and broccoli until they starve to death.
Dogs, sort of like bears, can survive as omnivores for a while. However, the metabolism of a cat is designed to subsist on a diet of nothing but meat. Has anyone ever seen a lion, cheetah, tiger, or any wild cat eating a potato? And you never will.
But even normal parents do this same thing sometimes. They feed their toddler diet no-fat/low-fat foods because they don't want the kid to be too chubby. The metabolism of a kid is 5 times an adult. They need all the fat/fuel they can get, within reason. This, of course, is why active kids can eat a whole bag of cakes and candy and not gain a pound.