The Wikipedia page that I linked provides a thorough explanation as to why corporations are considered citizens, so I do believe it directly answers the question. In this light, it was not intended as an appeal to authority.

Personally, I think the status quo is entirely and grossly unjustifiable. Citizens United needs to be overturned; corporations are not people and do not deserve the same rights as individuals. My opinion on that matter does not change the reality that they are considered such and only serves to confuse the matter.