Obesity in pets is 100% the fault of the owner, although pet food manufacturers don't make it easy.. I have 2 cats whose breed is well known for being commonly overweight but they're both at their ideal recommended by the vet. It did take effort to find healthier food for them and the correct amount, as well as build up their habits that they get as much as is in their bowl and nothing more regardless of the amount of complaining. It's the same logic as with humans, count the calories going in and adjust that - except pet food manufacturers don't often disclose even an estimate.

Outside cats are a divisive topic already though, in those cases it's arguable whether the owner is solely responsible for it all (due to the decision to let them out) or the other people who also feed the clearly well fed cat a 2nd and 3rd meal because they're trying to befriend it or it "looks hungry".

If we could just have a "don't feed someone else's pet unless you're asked to" mentality + people would research good food for their pets as a standard it'd go a long way imo and it seems safer than putting animals on drugs.