I didnt say it that its not cruel to cage dogs. I said its less cruel than caging a panda because they are more used to it.
I dont know if its fair to call it that, but dogs are probably still pretty "sad" about sitting inside doing nothing all day, especially when they are alone. If you have ever lived with a farm dog that was free to go out and wherever it wants 24/7 by itself, then indoor "incarcerated" dogs will seem like an empty shell to you.