Why did the majority say corporations and unions could make independent political expenditures?

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

The majority concluded that these restrictions on independent expenditures were unconstitutional because they infringed upon the right to free expression. The court maintained that transparency and disclosure requirements—facilitated by the internet—would be sufficient to protect the public and shareholders from potential corruption or undue influence, allowing citizens to identify the sources of political messaging and determine if elected officials were beholden to specific interests. By invalidating these federal restrictions, the court prioritized the protection of political speech over the government's interest in limiting the influence of corporate and union spending in elections.21