Government creates laws which permit or forbid corporations from doing things all the time. In this case, laws allowing corporations to restrict free speech are balanced against laws which forbid such restriction on private citizens, and the constitutional law which says 'government cannot restrict free speech' is interpreted to mean also government cannot permit corporations to restrict free speech. Not a hard concept, for a modern law scholar, or a constitutional law scholar. The idea that speech can be restricted by a private entity in the US is the stretch that originalists would have a problem with (e.g. you can yell fire in a crowded theater and policies to the contrary are prior restraint and unlawful by corporations or by governments).