I.e., shareholders hire directors to hire a CEO to hire employees to hire publishers to write or broadcast messages. Biological humans all. To hear the censorship fans talk, you'd think corporations were autonomous robots.
Suppose you got your wish and the Supreme Court ruled that the government can censor what it chooses to define as "corporate speech". How do you imagine that would be enforced? How do you think it is possible for the government to punish a corporation for uttering "corporate speech" other than by punishing one or more living breathing H. sapiens organisms for what some living breathing H. sapiens organism uttered?
I could imagine fines or the lose of some (if not all) of the corporate advantage
I.e., some of the shareholders' property is confiscated from them, or else a minority of shareholders are stripped of the limited liability protection shareholders in other companies are guaranteed because the government didn't like what those particular shareholders paid somebody to say. That plainly qualifies as punishing one or more living breathing H. sapiens organisms for what some living breathing H. sapiens organism uttered.