People in the USA have both the right speak as they wish and to associate amongst themselves freely for any lawful purpose. A corporation is nothing more than a group of people who are associating with each other.

If one person acting alone has a right to speak, how can that same right be denied to a group who have decided to speak together? And since “speech” in this context has always been interpreted to mean any act by which a message is conveyed to other people, it naturally includes applause, sitting quietly, donating money, etc, etc.