Phil --
I think you are over-emphasizing the ability to sue. Corporations have acquired certain rights of 'natural persons', but this is not really necessary to be a defendant of a lawsuit.
Here is a definition:
Corporation - Wikipedia, the free encyclopedia
The defining feature of a corporation is its legal independence from the people who create it. If a corporation fails, shareholders normally only stand to lose their investment (and possibly, in the unusual case where the shares are not fully paid up, any amount outstanding on them - and not even that in the case of a No liability company), and employees will lose their jobs, but neither will be further liable for debts that remain owing to the corporation's creditors unless they have separately varied this, e.g. with personal guarantees. This rule is called limited liability, and it is why the names of corporations in the UK end with "Ltd." (or some variant like "Inc." and "plc").
From the wikipedia article linked above:
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Proponents of corporate personhood believe that corporations, as representatives of their shareholders, were intended by the founders and framers to enjoy many, if not all, of the same rights as natural persons, for example, the right against self-incrimination, right to privacy and the right to lobby the government.
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The stronger concept of corporate personhood, in which (for example) First, Fifth, and Fourteenth Amendment rights have been asserted by corporations, is often traced to the 1886 U.S. Supreme Court case Santa Clara County v. Southern Pacific Railroad Company (118 U.S. 394).
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The problem is how broadly this is being applied now -- protections normally given to persons such as free speech, self-incrimination, equality. Corporations can be legal entities, but should not have the liberties provided for individuals. (Well, at least liberties we have when not under a Republican administration.)