I know that the topic here is fat cats however take a look at the link below.
Brothers found guilty over 'grossly' obese dog | UK news | guardian.co.uk
There have been several instances lately in UK about owners being prosecuted for having obese pets I am sure there has been one about cats but I can't find a link to it. There have been a few about rabbits.
My opinion is that it is cruelty unintentional or otherwise as it seriously reduces the animals quality of life, but I am sure others will have a different opinion.

Cheers

NCM
Nightcrewman Reviewed by Nightcrewman on . A very big kitty! Did y'all all see this story about the lost cat in New Jersey, Princess Chunk? How did someone manage to lose this animal?? 44-POUND CAT, PRINCESS CHUNK: -- chicagotribune.com Rating: 5