Outsider Art - Outside What?

Wild Hog Whirligig by David Butler

Outsider Art is a convenient term, but for whom? Who is 'outside' and why - and what does it mean for the Art?
http://www.artsmarttalk.com/3/post/2013/05/outsider-art-outside-what.html

A reflection on labels and meaning, prompted by the exhibit
'Great and Mighty Things: Outsider Art from the Jill and Sheldon Bonovitz Collection' at the Philadelphia Museum of Art