""I think making weighing voluntary is the ethical thing to do here. As with anything related to the human body, consent is important," Boston-based therapist specializing in eating disorders, Molly Robson LCSW, told The Reg."
Firstly, it should be perfectly possible to weigh every passenger and still do it completely ethically. Have a scale just at the point the boarding passes are checked, so each passenger is weighed exactly once. All the machine needs to keep are a list of weights, there is no need (and indeed there should be no way) of associating a weight with a person or any other identifier. There is even no need for the machine to display the measured weight, just an LED that blinks to say that the machine is done with the measurement, next please. If it's completely voluntary, the people opting out wil be those more sensitive about their weight, which completely invalidates the results.
Secondly, while it's nice to include the point of view of a " therapist specializing in eating disorders", the number of people who are overweight, or indeed obese, because of eating disorders are a very small %age of overweight/obese people. In the vast majority of cases it is a choice* to prioritise food over exercise, junk food over healthy food etc. If someone has an eating disorder, my commiserations, and exceptions can be made. Otherwise, if a passenger doesn't fit in their allocated seat, why do they think it's OK to overflow into the next seat rather than pay for a second seat or pay extra for a wider seat?
*or a habit, which has become a habit because of the accumulation of many hundreds of small and innocuous-seeming choices