Because political speech protection was designed specifically for real people to be free of reprisal for the content of their speech, not to hand a bullhorn to a nonhuman shell. Anything that can be funnelled through a corporate shell can more easily be said directly by the puppetmasters behind it, after all. Back to square one, indeed!
What I defend is Impartial. What You defend is Selective and Arbitrary. It also opens a Pandora's Box of Imagined Government Powers, in a way You are currently blind to. If the Venues are Corporately Controlled such as Networks and Publishers, Airways, be it Internet or Phone, the Totalitarian Statists, can make new intrusive claims on how The control Our Liberty. Book Censorship was a part of this Law Suit. The Government Power to Censor, Restrict, and Deny Publishing. Books, Movies, Documentaries.
I'm anti Monopoly, and am not a big fan of Corporations Goldcatt. I've studied more than my share of articles on Your side of the Argument, I probably stand with You on most issues of Corporate Abuse. Speech in self defense is not one of them. Speech, that informs, that is verifiable, deserves to be allowed.
The Abuses You are concerned about seem more related to what goes on behind the closed doors of Corporate / Government Negotiation and are totally separate of the issue of Public Voice.
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
Neither Persons or Corporations are mentioned here, in relation to speech.