Corporations are people?

I've been mulling over the Citizens United vs. Federal Election Commission decision last week.  The part that I find most worrisome of this whole question is not the issue about campaign advertising, which hopefully can be dealt with by legislation, but the idea that a corporation is a "person."  This is nothing new - this appears to have started in 1886.  The problem, is that as I see it, this really stretches the definition of person in American constitutional law.  Going back to the Declaration of Independence, rights are inalienable - granted by God - to those whom God has created.  (You can equally substitute Nature here if you are atheist.)  A corporation, by definition, is "unnatural," that is, something that does not exist in the natural world, but is a construct of secondary human law.  It is very hard for me to imagine the founders thinking of corporations as persons.  They certainly had those, but did not go out of their way to grant them rights, nor did they to any other organization created by human law, such as marriages or fraternal brotherhoods.  Corporations might (and should) be granted certain rights by legislative process.  But to insist that their rights exist by default, that they are in effect as inalienable as those granted to human beings, seems to me to be a devaluation of the entire achievement of the Enlightenment vis a vis human rights.