The "rule of law," however, means that if the Constitution or other laws bar X, then X is not allowed regardless of how many good outcomes can be achieved by X. That was true for the "crisis" of Terrorism, and it's just as true for the crisis of corporate influence over our political process. Whatever solutions are to be found for either problem, they cannot be ones that the Constitution explicitly prohibits. That's what "the rule of law" means.So, for example, this means that just because torture gets us actionable intelligence (and even that is debatable), because it's prohibited under the Constitution via our ratification of the Geneva Conventions means that torture is illegal and never justified under any circumstances.
However, Greenwald's central argument in this post is that none of the nine justices advocated a view that corporations are not persons entitled to free speech, and that how a corporation spends its money on political issues is not speech. On the contrary, even the dissenters in the case did not argue those points. What they argued is that there is a "compelling state interest" in restricting a corporation's free speech rights: namely, the corrupting influence of corporate money, which required regulating how they got to spend their money (which led to the creation of PACs into which corps could direct their money). But that objection, as Greenwald points out, leads to one restricting any type of corporate speech, not just the political or ideological type. Restricting corporate speech because of the corrupting influence of money can lead a state to restrict advertisements for food products that make us fat, since it can also be argued that there's a compelling state interest in fighting obesity. (I am aware that we currently regulate advertisements for cigarettes, but I'm not so certain that such regulations are constitutional. In fact, I'd be a bit surprised if Big Tobacco didn't try now to argue in court that restrictions on their advertising should be lifted.)
The central argument of many people opposed to the Citizens case seems to be that big corporations and their bottomless pit of resources will tip the balance of power toward those who will promise to further the causes of corporations (or cause, meaning profit). Well, maybe. But while the Constitution does not expressly declare that corporations are persons with rights, case law throughout US history does. And corporate speech which advocates political views of any particular slant should not be restricted because the First Amendment prohibits it.
This is not to see that I disagree with the argument that the corrupting influence of money does have an impact on the outcomes of elections. I agree with that. But the law does not allow the state to restrict speech because we think it will create an undesirable result (with some narrow exceptions for violence or even narrower ones for obscenity). I think this should embolden individuals to coalesce more tightly around their common causes and to vote with their wallets. If, for example, a corporation openly argues a political point you abhor, don't patronize that product, organize boycotts against it, write op-eds, post on blogs and other websites, organize rallies in front of their corporate headquarters and invite the media, etc. That's right: the Supreme Court decision in the Citizens United case should get us off our asses to fight for the causes we champion.