Southwest is charging for the seat not the weight. If you overflow your seat and no one wants to sit next to you then Southwest is losing money from a seat they could sell. They might have been able to have the kid sit next to his mom and get squished though and not charge her. All this whining seems like Democraps.com.
But the seats aren't big enough to start with. They should advertise "we sell 12 inches of space..if your butt is bigger than that it'll cost you."
I think they should have to provide a seat that will fit the obese person.