I think part of the difficulty is the terminology. If you have a co-operative, in this case for the purposes of the tax rules, it would be treated as a corporation, at least for the small business deduction rules. But if you're talking more colloquially and not in the specific technical sense that you have in the tax rules, they're carrying on.... I shouldn't say “carrying on business in common” because that's the technical legal test for interpreting a partnership, but if you have a number of people operating through a joint venture or in a somewhat co-operative fashion, then that's a different thing.
As I recall, the concern from a lot of medical practices was that, while they initially thought they ought not to be considered partnerships for the purposes of the rules, they might be considered partnerships legally, and, as such, would be into the multiplication of the small business deduction rules that way and not because they were legally organized as a corporate co-operative within the purposes of the small business deduction rules.