No one has suggested that you should. People have suggested that the corporation acting as such, including through its paid agents should not have the same rights as natural persons, acting alone or collectively, but that various classes of corporations should have constraints appropriate to their class.
One of the many reasons offered for this is that corporations are creations of government granted the use of the special form and legal fiction of personhood adn subsidized with various benefits with the purpose of achieving public benefit.
> In that respect, what makes corporations different from unions or advocacy groups like Greenpeace?
Unions and advocacy groups quite often are corporations. "Unions" and "advocacy groups" are descriptions of functions, "corporation" is a description of legal structure.
Who will decide what class a corporation falls into?
I get the uneasy feeling that, practically speaking, corporations expressing certain approved political opinions or supporting approved political candidates will have the least constraints.
Because you carry in your wallet a card that declares you a member of a corporation, you are not granted any rights, personally.
Near as I can find, corporations don't have carte blanche to give unlimited money to an individual campaign. In fact, corporations cannot give any money to an individual candidate's campaign.
Corporations can give money to Political Action Committees, but are limited by amount to the same as real people.
Corporations can give money to SuperPACs, but so can real people.
http://www.fec.gov/info/contriblimitschart1516.pdf
http://www.fec.gov/pages/brochures/citizens.shtml
If I am wrong, I welcome corrections, but I think you're off on what corporations can and cannot do.
So no more collective bargaining?
Why is it I lose my right to speak if I'm in a group? In that respect, what makes corporations different from unions or advocacy groups like Greenpeace?