If a passenger only has half a seat, it is only fair to have them pay half.
The average size human can easily fit in a normal size seat. This is not at all offensive as it is only bringing justice to the regular sized persons who care about their body weight.
It's no fun to have only half a seat, especially when when the person next to is so fat that they are spilling into your seat. People are entitled to personal space and I think that everybody should respect that no matter their size. Also they could upgrade their seat to a larger seat.
When purchasing an airline ticket, you are purchasing the right to use the space allotted on an airplane. If you are so obese that you cannot fit in the space allotted to you, then you must either find a different mode of transportation, or purchase additional space. It may be offensive, but there are other people traveling with you that must endure having their space constantly invaded.
To charge extra because of a persons size is completely prejudicial, and would certainly offend people. It's not as if a persons size would cost the airline more to transport them to their destination.Also it was raise alarm to other potential clients as to when they would be targeted for unreasonable extra chargers based only on appearance.