With surging fuel prices, airlines are trying to cut their costs, and perhaps pass some of this burden to passengers. Some airlines are starting to enforce rules that over-weight passengers should purchase extra tickets. Although such practices trampers on touchy issues as sort of discrimination based on people's weight, it probably has some economic justifications. I have witnessed some extremely "well built" passengers trying hard to fit themselves into the economy class seat, and the seat handles were just too tight for them. It would make sense to have them purchase two tickets, so that they and their neighbors can all sit comfortably.
Of course, where should airlines draw the line between overweight and normal passengers? Some are require you to be able to fit within the seat belt. But then just how long should the seat belt be?
Ultimately, it is yet a another reason for people to watch their weight, and perhaps loose some of the extra fats :-)