The protest against corporate personhood is deeply incoherent. If you impose obligations on a corporation and hold it responsible for not meeting that obligation, you are treating the corporation as a thing with agency. Why shouldn't that corporation be able assert rights against this imposition?
What are the defining characteristics of a person? What makes one thing a person and not another thing?
Absent a religiously inspired answer, anything you come up with will an inadequate answer. It will be over-inclusive or under-inclusive. A genealogical investigation will reveal that our concept of personhood is inconstant. An anthropological investigation will reveal that personhood is culturally conditioned and varies between peoples.
Personhood is a social construct. You naive metaphysicians objecting to corporate personhood need a better argument than "corporations aren't people."
There are good theological explanations of personhood. There aren't good secular explanations that would disqualify the application of personhood to corporations for certain purposes.