underneaththeash,
"I can't see the fuss either, being fat isn't healthy or attractive."
Being fat and unfit may not be healthy; certain people (restricted by unrelated disability, for example, or due to medication side-effects) struggle to improve that fitness and/or lose weight for very real reasons.
You might want to add "in my view" to your fat equals unattractive comment. Plenty disagree.
This short piece (easy to read, very accessible) from
"Popular Science" discusses the fat fit issue fairly from health viewpoints, IMO.
Or
(for those wanting in-depth, referenced, hard science)
This paper, in "Journal Of Sports Science"
- [Research references]
Abstract:
"...Whereas “overweight” implies increased risk, it is in fact associated with decreased mortality risk compared with normal weight.
Another paradox concerns the observation than when fitness is taken into account, the mortality risk associated with obesity is offset.
The final paradox under consideration is the presence of a sizeable subset of obese individuals who are otherwise healthy. Consequently, a large segment of the overweight and obese population is not at increased risk for premature death. It appears therefore that low cardiorespiratory fitness and inactivity are a greater health threat than obesity, suggesting that more emphasis should be placed on increasing leisure time physical activity and cardiorespiratory fitness as the main strategy for reducing mortality risk in the broad population of overweight and obese adults."
Back to the main topic, though, why won't Skinnypigs give a definitive answer as to the logic behind their weird, judgemental, public advertising being so at odds with their supposedly 'empowering' atmosphere at the actual classes?
"Overweight" people looking to improve their fitness levels with them might well now assume that they'll be treated like a pariah who could surely only be there to lose weight, and guilt-tripped until they do.
Or be put off the whole fitness/health thing altogether.
Devotee:
"Every fat man-hating, humourless, only-cats-for-company, lazy, Feminazi loon person just needs to get shifting and put some effort in to exercise. They'll boost their self-esteem and confidence once they realise what Skinnypigs is all about. We're motivating, yet fun."
'Overweight' person:
"Well I could do with support in a nice class atmosphere. I'm fed up with ignorant teasing and jokes whenever I'm trying to work out in the park. Others there will understand, I'm sure..."
"Oh. Clearly not, then".
Kind of ironic. 
[Have said all along I don't doubt for one minute how life-enhancing and all-round-bloody-fabulous Skinnypigs might be to their members (the pr machine has gone into overdrive on damage limitation telling the world and its wife so)...am just really not impressed by their wider message].
Here's a pic of some actual 'skinny pigs' to cheer us all up! 