I fast my raw fed (prey model) adult dogs once every other week. We do the gorge and fast method so it's not as if they're being starved. Fasting has nothing to do with keeping a dog's weight down.
Sometimes they self fast, the spaniel in particular. He'll refrain from eating for a day or two, but eating has always been a chore for him. Playing nonstop has always been his passion in life.
It's easy to dismiss something you're not really informed about as useless or cruel. It's always best to educate oneself about the topic before jumping to conclusions about why some people choose to do the things they do with their own dogs. Even if you've done your homework and still disagree with it, there's always a way to share your opinion without shooting others down.
Agreed with what others have said about what one's beliefs about cruelty. How about you consider the people on the other side of the spectrum who strictly feed their dogs a prey model raw diet... to some of them, feeding a dog food that isn't species appropriate like kibble is selfish and cruel...