A very good explanation, courtesy of Ilya Shapiro, and Caitlyn McCarthy. Behold the abstract:
Corporate participation in public discourse has long been a controversial issue, one that was reignited by the Supreme Court’s decision in Citizens United v. FEC, 130 S. Ct. 876 (2010). Much of the criticism of Citizens United stems from the claim that the Constitution does not protect corporations because they are not “real” people. While it’s true that corporations aren’t human beings, that truism is constitutionally irrelevant because corporations are formed by individuals as a means of exercising their constitutionally protected rights. When individuals pool their resources and speak under the legal fiction of a corporation, they do not lose their rights. It cannot be any other way; in a world where corporations are not entitled to constitutional protections, the police would be free to storm office buildings and seize computers or documents. The mayor of New York City could exercise eminent domain over Rockefeller Center by fiat and without compensation if he decides he’d like to move his office there. Moreover, the government would be able to censor all corporate speech, including that of so-called media corporations. In short, rights-bearing individuals do not forfeit those rights when they associate in groups. This essay will demonstrate why the common argument that corporations lack rights because they aren’t people demonstrates a fundamental misunderstanding of both the nature of corporations and the First Amendment.
(Thanks to Larry Solum for the pointer.)