In the Washington Post, Boston College lawprof Kent Greenfield clears up some misconceptions:
Citizens United did not hold corporations to be persons, and the court has never said corporations deserve all the constitutional rights of humans. The Fifth Amendment’s right to be free from self-incrimination, for example, does not extend to corporations. … Humans gather themselves in groups, for public and private ends, and sometimes it makes constitutional sense to protect the group as distinct from its constituent humans.
The question in any given case is whether protecting the association, group or, yes, corporation serves to protect the rights of actual people. Read fairly, Citizens United merely says that banning certain kinds of corporate expenditures infringes the constitutional interests of human beings. The court may have gotten the answer wrong, but it asked the right question.
Another reason to protect corporate rights is to guard against the arbitrary and deleterious exercise of government power. If, for example, the Fifth Amendment’s ban on government “takings” did not extend to corporations, the nationalization of entire industries would be constitutionally possible. The Fourth Amendment prohibits the FBI from barging into the offices of Google without a warrant and seizing the Internet history of its users. A freedom of the press that protected only “natural persons” would allow the Pentagon to, say, order the New York Times and CNN to cease reporting civilian deaths in Afghanistan.
The actual Citizens United case, as distinct from the later caricature, was over whether the government had a constitutional right to punish private actors for distributing a video critical of a prominent politician (Hillary Clinton) before an election, which helps explain why the ACLU and many other civil libertarians took the pro-free-speech side. More: Caleb Brown at Cato.