No one likes getting nickled and dimed. While they are at it, weigh everyone before boarding, to properly charge them for how much extra fuel they consume beyond the base charge. Then heavier people won't be using more than their "fair" share. Except now you've pissed off even the super skinny people who have to deal with the "transaction cost" / extra time involved, who were perfectly willing in the past to perhaps pay slightly more than if they were heavier because it simplifies the experience. Heavier people also think that way, not "oh boy I'm getting a good deal flying at the same price as this lightweight".