This is an interesting question I had not thought about until I watched the documentary The Corporation, one of the most successful Canadian documentaries ever. (You can watch the whole documentary here on YouTube, provided by the makers of the film. In particular, see videos 1 and 2.) It was shown in my business ethics course.
The Fourteenth Amendment, ratified in 1868, was a Reconstruction Amendment aimed at advancing the rights of blacks. One of the most important statements it makes is, “No state shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws.” So, fundamentally, at issue were civil rights and the citizenship of slaves and black people.
Naturally, we would expect that cases being brought to the courts on the basis of the Fourteenth Amendment would be about the rights of blacks and their rights as citizens of this country, since that’s the purpose of the Amendment and why it was ratified. But this isn’t the case. According to Doug Hammerstrom, “Of the 150 cases involving the Fourteenth Amendment heard by the Supreme Court up to the Plessy v. Ferguson case in 1896 that established the legal standing of ‘separate but equal,’ 15 involved blacks and 135 involved business entities.” Backwardly, it was not the blacks who were winning their cases for a more a just and fair society wherein they could secure and protect their rights (they were, contrastingly, being systematically oppressed) as citizens, but the corporations who garnered individuality as persons under the Fourteenth Amendment. (See, for example, Supreme Court case of Santa Clara County v. Southern Pacific Railroad.) There is robust scholarship on the issue of corporate personhood, which I have only begun to delve into.
Through a series of Supreme Court decisions, corporations became individuals, like you and I, who can carry many of the same functions, such as trade, buy, sell, sue, be sued, lobby, etc. This is a particularly important fact when it comes to liability. So, while corporations could now enjoy many of the same rights under the Constitution as any other person, they were not bound similar restraints, thanks to the many deregulation movements this led to. The ubiquitousness of the corporation in today’s society can be traced back to these decisions. We live in the corporate era, which is distinctly different from how business was previously regarded. With the corporation as a dominant institution in modern society, there are major moral implications to be considered. I would suggest you view the documentary I mentioned above for a discussion on these issues.
However, if we going to treat corporations as individuals, as people essentially, then we should also hold them to the same social and moral standards as we do flesh-and-blood people. Or, alternatively, we can change our perception of corporations. Perhaps, as others have suggested, they should not be thought of as persons under the Fourteenth Amendment, but perhaps as tools to promote the social good and needs, as they once were, being wholly accountable to the people. Unfortunately, the documentary does not elaborate on what the functions and properties they think the modern corporation should possess. I wish they did.
Update: Please see a relevant post regarding a Supreme Court decision on corporate personhood.