The Supreme Court has ruled that it “does not disable the government from taking steps to ensure that private interests not restrict...the free flow of information and ideas.”

See the case here: https://www.law.cornell.edu/supct/html/93-44.ZO.html

The full text: "The First Amendment's command that government not impede the freedom of speech does not disable the government from taking steps to ensure that private interests not restrict, through physical control of a critical pathway of communication, the free flow of information and ideas."