1.) The American public is smart enough to make the decisions necessary in a democracy (The Wall Street Journal Editorial Board 1/22/2010):
In last year's oral argument for Citizen's United, the Court got a preview of how far a ban on corporate-funded speech could reach. Deputy Solicitor General Malcolm Stewart explained that, under McCain-Feingold, the government had the authority to "prohibit the publication" of corporate-funded books that called for the election or defeat of a candidate.
2.) The American public is too stupid to be entrusted to make the decisions necessary in a democracy (Desperado, Something different, Chron.commons @Chron.com 1/22/2010):
“Under today’s decision, insurance companies, banks, drug companies, energy companies and the like will be free to each spend $5 million, $10 million or more of corporate funds to elect or defeat a federal candidate — and with that power, influence the candidate on issues of economic importance to the companies.”
What those who wish to censor political speech with which they disagree fail to point out is that they judge themselves smart enough to make those decisions. The 'idiot' part is just reserved for anyone who views the world from a different perspective than they.