Once again Mr. Private Pilot... you obviously have no clue about how non-rev situation works, nor does it seem you were ever left behind as a revenue passenger or a non-rev. I'll also skip the weak attempt to insult me as I can tell I'm talking to a 19 year old know-it-all.
Once ALL the paying passengers have boarded and that includes the large people, then you get non revs aboard in order of priority.
Once again, airlines sell SEATS, if your rear end is too big to fit into one so you require two, you are in effect ripping off an airline of putting another paying passenger in that seat especially if the flight is at or near capacity.
Let me put it to you another way...
You paid full fare, say $500 for your ticket on a flight that's booked to capacity which means no seats left - every seat is taken. Pre-boarding takes place, when all the people who need "a little extra time" to get set, then you go in there with the rest of the crowd.. and you see a fat lady taking both seats and you can't sit in there. The F/A's gave her the seatbelt extension so she could buckle up. Yet, now you can't fit in your seat, and you are getting denied boarding because of no seats left. But you HAVE to make your flight. Now what? You still get the boot, the fatty goes... so now the plane pushes from the gate and you are left behind fuming at the gate agent. You want to be on the next flight that leaves the following day in the morning. So now, the airline, in the interest of customer service has to accomodate you as well i.e. provide you with hotel room and a meal, in addition to give you something "for your troubles." So now, instead of airline making a profit on your ticket like it is supposed to, they lost revenue on the original flight because two seats were taken by ONE person, they pay you for your trouble, put you in a hotel, they are putting you on ANOTHER potentially full flight, and potentially bumping ANOTHER paying passenger all because of one fat person who could not fit in her seat. Now they'll have to accomodate the other guy they bump and the cycle goes on and on.
Airlines are not public utilities, they are FOR-PROFIT businesses, therefore it is entirely within their rights to conduct business as they see fit. If the people who cannot fit in a seat do not agree with airline's ways of conducting business they can either try on another airline or they can take the bus.
ALOHA!