Why did the Supreme Court say independent expenditures do not create quid pro quo corruption?

In the case of Citizens United v. Federal Election Commission, the Supreme Court majority ruled that independent expenditures do not create quid pro quo corruption because, by definition, they are not coordinated or prearranged with a candidate or a campaign.1

The Court reasoned that because these expenditures are made independently, they cannot result in the direct exchange of money for votes that characterizes quid pro quo corruption. While the majority acknowledged that such spending might lead to greater access to a candidate or help ingratiate a corporation with them, the Court explicitly stated that "ingratiation and access... are not corruption". Consequently, the Court concluded that the government’s interest in preventing corruption or the appearance of corruption was not narrowly served by banning these independent expenditures.1