I have no objection to the charge. At least my weight of recent is going in the direction of no-fat-fines...thank goodness!
There are supposedly people who are obese for 'medical' reasons through no fault of their own. Perhaps they'll get their extra seat for free upon medical/doctor proof. Everyone else - extra seat please.
Mind you, even when I was heavier, I still fit in a 737 or Dash 8 seat without spillage into the next seat. Naturally, I sat a bit more 'tightly' compared to if I was sitting in any other seat; whilst I could really spread out if I wanted to, I'm mostly more paranoid about invading someone's space compared to that person being paranoid about my invading their space.
The way things are going, one day they'll resort to total weight basis, i.e. weigh you and your luggage.
The whole argument seems very anti-libertarian and highly discriminatory, though. How did some airlines manage to pass such a rule, especially in North America in the guise of the (in)famous Canadian court ruling?