There's a difference between weighing machinery and luggage and putting a price on their haulage, and weighing a human being and putting a price on their haulage based solely on their weight. It all boils down to the difference between someone being coldly logical and actually being compassionate.
I'm a big guy, so clearly don't subscribe to the usual way of referring to people who are overweight (bloaters, puddings, lard buckets all the usual that is bantered around in this thread.) Neither do any of my friends, many of who are the exact opposite of the types of people this thread is referring to. Personally, fat or not, smoker or not, crack addict or not, I don't like being derogatory to somebody, even if I don't agree with how they live their life. Encouragement is always better than telling someone they suck...
That aside I fully understand peoples reaction to very large people on planes. I say I'm large, I'm 6ft 3 with a wide frame, but according to my BMI (bad measure I know) I'm not obese, middling overweight (and working on it). So I fit into airline seats with no real problems other than legroom, and don't "spill over".
To get somebody to stand on a set of scales, with their bag, and then charge them in a public place is nothing but cruel. Those of you who aren't overweight will say "well it's their fault", and while I agree, that doesn't mean everyone has a right to look down on these people. However, if an airline, while booking, offered someone the option to purchase two seats, with guidelines (i.e. if you are wider than X we suggest you purchase two seats for your flight or you may not be able to fly if a flight attendant deems that you should have), then that is fine and tbh should be encouraged.