They do in Canada now, says Swaraaj Chauhan, who illustrates his post with an astoundingly apt photograph. Hipopinion has an even more distressing photograph. (Canada might want to recognize a right not to have one's obesity appropriated to amuse and teach lessons to the normal-weight community.)
If you get a free extra seat now, won't people be clamoring to be considered one of the truly obese? Does some government agency certify that you are fat to the point of disability and thus entitled to accommodation? Or is this just a matter of airlines telling some people they'll have to pay for a second seat -- and now, they're being told they will need to give that seat for free? If the latter, some of the chubbier passengers may decide to sprawl strategically to extort extra space from the airline. If the plane is full, who gets bumped?