At what point is a person too large? Who decides? What kind of grievance process and review are available to those who feel have been unfairly singled out?
As Jason alluded to, there was a time not that long ago when a seat in coach wasn't pure torture for anyone bigger than a ten year-old. I'm not a big man at all (5'6, 160), and I still require a bottle of benzos before flying these days.
Personally, I see it as making accommodation where possible. We do not charge someone with a disability more (no, I don't think obesity is a priori a disability), even though s/he might well monopolize the attention of much of the flight crew. Personally, I'd much rather see much higher costs for traveling with a loud, crying, hyperactive or otherwise obnoxious child, but I don't see mach danger of that happening, either.
Canada ruled on this same issue about a month ago in favor of the obese passenger and against Air Canada; you knew it would trickle down south sooner or later.