Many citizens have much more money than many corporations. Is it ethical to let citizens spend as much as they please when their interests compete with those of corporations who up until now could not spend?
Perhaps not, but what about corporations who have a much greater amount of money than any private citizen (or organization) could ever hope to hold?
The principle behind the decision today is that the First Amendment does not permit a distinctions to be made based upon the identity of the speaker. It's all about the "marketplace of ideas": the more speech that is introduced, the better.
I can agree with this in principle, but when some speakers have the financial ability to introduce a disproportionately large amount of speech in pursuit of their own self-interest, even when that self-interest hurts the citizenry at large, are we better off as a result of that speech? If that speech causes a candidate to be elected who will act for the needs of the few (i.e., the corporation) are his or her constituents better off, or is the notion of representative government failing?