That is just plain cruelty. Apart from the host of health problems overweight cats can be susceptible to, when they're that big they can't carry out normal every day activities like washing and running around.
I think that given the difficulty in getting cats to lose weight, it's much better to not let them get overweight in the first place. I think we've got so accustomed to obesity being a part of human society that it's affected our judgement of what is overweight and what is normal, and we carry that over to our pets. Jaffa and Mosi have never been at all overweight but I'm lucky in that they dont' go outside so I can control what they eat.