Corporations are NOT people, nor are they entitled to the Natural Rights of humans.
They are created entities, given, BY STATUTE, privileges equivalent to SOME of the inherent, unalienable, RIGHTS of NATURAL people, for the specific purposes of allowing them to own property, pay taxes, and to be sued should they cause harm.
Earlier SCOTUS decisions (over the course of decades) attributing more and more of the characteristics of natural persons TO corporations, are based on a perverted reading of the 14th Amendment to the Constitution.
While there should be no legal limit to the amount natural, flesh-and-blood, individual HUMAN persons can donate to political causes, there is ABSOLUTELY NO CONSTITUTIONAL JUSTIFICATION for ONE PENNY to be donated by the artificial creations called "corporations" -- or PAC's, or (most especially) UNION's, either.
As for those who claim that corporations (or unions) are "merely" assemblages of natural persons -- no. Not in this circumstance. Any of those natural persons who are associated with a corporation may surely give their OWN money -- but your assertion tacitly implies full consent and agreement of EVERY employee, director, and customer.