Here, in Boston Review:

The better argument in favor of limiting partisan political spending by large, publicly traded corporations rests, ironically, on the fact that corporations are made up of people. Under current law the actual owners of corporations—their shareholders—have little say in how corporations make decisions in the political arena. That corporate managers might spend corporate funds not to maximize the shareholders’ welfare but to maximize their own is a very real danger. Many shares are owned by mutual funds and pension funds that in turn are owned by individual citizens who often have political convictions that go beyond maximizing the profitability of the corporations whose stock forms part of their retirement savings. What is more, those political commitments may be sharply at odds with the economic interests of the corporate managers who are making decisions about corporate political spending. The law should not force citizens to forgo beneficial investments in order to avoid subsidizing their political opponents.