Why did Citizens United v. Federal Election Commission protect independent political spending by corporations and unions?

In Citizens United v. Federal Election Commission (2010), the U.S. Supreme Court ruled that laws prohibiting corporations and unions from using their general treasury funds for independent political advertising violated the First Amendment guarantee of freedom of speech.1

The Court’s decision was rooted in the principle that political speech is central to the meaning and purpose of the First Amendment. By invalidating restrictions on independent "electioneering communications," the Court held that the government cannot suppress political speech based on the speaker's corporate or union identity. This ruling built upon the precedent established in Buckley v. Valeo (1976), which recognized that spending money is an indispensable instrument for effective political communication in a mass society.12

The Court concluded that such expenditures are a form of protected speech and that the government lacks the authority to restrict them simply because it deems the spending "wasteful" or "excessive," or to prevent the influence of special interests in the electoral process.12