In the time when the 1st amendment was written (which is not a holy document that decides all morality. Nor is the U.S. the center of the universe) corporations as we understand them did not really exist. The people that were concerned about free speech could only conceive a hostile government of King George could really suppress them and the people willing to hear their dissident ideas. They were rich local gentlemen that had public squares and private meeting rooms. It did not occur to them that they could be silenced by the local placard maker forming a cabal that would deny any attempt to express themselves.

Times have changed and the public square is dead.