I think much of your argument boils down to the premise that you are just getting a ticket to go from X to Y when you purchase transportation, but there is no guarantee of space. I disagree with this premise.

When you are talking about public transportation like a bus, yes, you are not guaranteed a "seat," so if you have to stand the entire time or are cramped, then that is too bad for you. But when you purchase a ticket on an airline, you purchase a "seat." Sometimes that includes extra amenities like first class, comfort plus, row vs. window vs. middle, etc., but it always includes at least a seat on which to put your butt. If you showed up with your ticket and they said, "You weren't guaranteed a seat, so you will have to stand for the flight," then you would be rightly upset.

So with your "seat" how much room are you guaranteed? Well, for those who are purchasing the same class of ticket, the logical argument is that you are purchasing your own seat, plus the immediate space in front of your seat. The arm wrests are a "free for all." If you wanted to put your elbow over your seat and into the next person's seat, that is wrong, because they purchased the seat. If you want to put your drink on someone else's seat because it is too cold to hold, that is wrong because it is someone else's seat. So, if someone else wants to take their large/fat legs and spill them over into the space immediately in front of you, that is also wrong in my opinion. This also goes into the basic argument of whether or not you "own" the space behind your seat so do you have a right to recline your seat or not, but that is a completely separate issue and I've heard strong arguments on both sides.

Where I do agree with you is that this is often borne out of anger/disdain for fat people. I have a friend who is 6'11'' and seats are adequate for him either, yet he never gets dirty looks because his legs are too long. No one assumes, "he is lazy" or has no self-control. But when it is an overweight person, we do tend to blame them.

However, you can still love or not judge a person, while allowing them the natural consequences of their actions. If you are on a boat or small plane with weight regulations, there is no judging involved. They will just say, "Hey you, go to the other side of the boat to balance things out." It isn't personal, they just don't want the boat to capsize. I've seen the same thing with tiny regional planes. So, if you are extremely overweight and can't fit in your designated space, the natural consequences (without judgement, mind you), are that you either purchase an additional seat to accommodate the space you take up, or keep your legs glued together and are miserable for 3 hours because you don't want to pay extra. It ain't personal.