right, you should never ever have to deal with "icky" people encroaching on your personal space that you paid hundreds of dollars for in a plane.
so the policy came about because of many, many, many non-fat people complaining to the airline
a) after the flight
b) with no witnesses or requirement for witnesses
c) without informing the flight attendant
d) without confronting the alleged offender
e) with no physical measurements of the extent of the offense
and there is no way for the average customer to follow the policy (Actually, there is - don't be a fatty)
even if you make sample airline seats available to the general public to try out to determine if they need an extra seat, seat sizes differ according to aircraft!
if you fit in the sample seats, what if, on the day of your flight, you're switched to a smaller plane with smaller seats?
and the policy is supported, as we see here, based on legitimate experience