The proof the corporations are people is simple: Anything you can forbid a corporation to do, its owners can do in some other form. A corporation is a legal fiction that allows its owners privileges not enjoyed by individuals, most notably, limited liability for contracts. A corporation may also have special tax provisions under state or Federal law. So, for example, a jurisdiction with the power to regulate individual contributions could require that any contribution by a corporation be treated as given by its shareholders in proportion to their voting rights, and that if the corporation cannot figure out who those people are, that's their problem.

The Internal Revenue Code denies deductions for certain political expenses incurred by businesses. Those expenditures may clearly be "ordinary and necessary" business expenses, but they are still non-deductible. An argument in favor of that rule might be that individuals cannot deduct such contributions, and corporations, wait for it, are just groups of people pooling resources.

Every so often, someone proposes an amendment to ban "flag-burning." OK, then, define "flag." If there were such an amendment, flag-burners would quickly find something else to burn that STANDS FOR the flag. How different from the official flag would that thing have to be not to be "the flag"? Could the shade of red be changed? The size of the stars? The order of the stripes? It's a fool's errand, like banning corporate contributions that a group of people could have made if put to the trouble of making them.