You are correct, but the GP talked about corporations which are creations of law (passed by our government) and not of simple association like the simple business you describe. These laws specifically give advantages to corporations and other legal entities which would not exist outside the law (again, passed by and enforced through our government). The greatest of these is, of course, the limitation on pass-through liability for acts of the corporation to its shareholders/owners. Nowhere in the Constitution is the right to create such entities explicitly given to the government, yet many folks seem to think their existence is just fine with them as is the shield from liability afforded to their owners (which is contrary to our legal notions of personhood and legal responsibility).
I'm fine with unlimited corporate contributions to political candidates so long as the government does not provide a protected status to corporations. Feel free to form business associations. but understand that each of the members of these associations will be fully, personally liable for all actions and debts of the association. What's that? Without protection from liability there will be no corporations? Fine by me. Until that point, entities whose existence is solely a matter of law should realize that the law can constrain as well as benefit them.