I don't think there's a 'must', some level that all models have to be willing to go to to be accepted in the industry. Fashion models surely aren't hired for their nudity (although they can make good subjects for it) and that's why fine art models exist (of course, fashion photographers typically aren't fine art photographers).
as for Terry Richardson, there's a little bit of controversy behind him and his methods, although obviously a lot of it is based on hearsay evidence which can be unreliable.
However he works though, I think by and large he's just a **** photographer to be honest. Utterly uninteresting, clearly driven by aestheticism of subject matter rather than composition, than of the photograph itself and, editorially, fairly devoid of interesting ideas. He's predictably popular but I find his work tedious and banal.