There isn't a ban on independent expenditures by corporations because such a ban would unconstitutionally abridge of the right of freedom of speech and of the press. It would abridge the rights of (1) the individuals who jointly own the corporation and (2) the individuals who may wish to hear the speech whose dissemination is being restricted.
Corporations don't have a constitutional right to even exist, but provided that they do exist, Congress cannot enact a content-discriminatory restriction on speech funded by corporations.
The people you're arguing against are taking a utilitarian view of the problem. You're arguing from unexamined principles. Provided that companies exist as legal persons, shouldn't they be allowed to vote too? What about incarceration in the event of wrongdoing (manslaughter)? Isn't the treatment of corporations as arbitrary in these cases as in the one you refuse to make?
Clamping down on corporate political speech would hardly restrict the speech rights of individuals. It would simply force organizations wishing to fund political activities into funneling the money through individuals with no legal recourse if the funds ended up used for something else. And it would increase the cost of the activity as compared to the present since individual recipients would be taxed for the additional income at progressive rates depending on the amount spent.
Nothing I've said relies on the legal fiction of corporate personhood. I was careful to speak only of the rights of natural persons.
>Clamping down on corporate political speech would hardly restrict the speech rights of individuals.
I respectfully disagree. If corporate political speech were unprotected, then Congress could have outlawed the SOPA protests by reddit, Google, and other corporations. Do you not feel that this would constitute an abridgement of the rights of the people behind reddit and Google to help inform the public about the nature of these bills?
If a group of individuals can join together and form a corporation to publish a book on almost any subject, but are specially prohibited from publishing a book endorsing or opposing a candidate or bill, then it seems to me that such a prohibition would clearly be an unlawful violation of the First Amendment, because it discriminates against certain speech based on the content of that speech.
If this is a question of principle you've surely already lost; there are plenty of non-controversial areas in which organizations are afforded greater or lesser rights than individuals based on their functional role in society: non-profits and charities are expressly forbidden from political campaigning, while the New York Times can legally publish Wikileaks in a way that Julian Assange apparently cannot.
I'm sure a sensible law can find a way to reduce soft-money flows without wiping out forums and newspapers. Nor is anyone saying that organizations cannot express political viewpoints. The question is about the corrupting influence of soft-money flows and if the situation is too complex to allow for that, it is surely too complex for limited liability corporations to exist in the first place.
Corporations don't have a constitutional right to even exist, but provided that they do exist, Congress cannot enact a content-discriminatory restriction on speech funded by corporations.