No, you can have one voice broadcasting, and as long as it's still one very loud voice, it's only one voice. This is part of the psychology of how people measure veracity of information: through apparent public consensus. So you can have CBS broadcast something, and that's fine, but it becomes problematic when CBS buy themselves a thousand Twitter bots with names like "Joe Smith" or "DankMemes88", thus giving the false impression that thousands of people are all in consensus.
I don't agree that a broadcaster has a responsibility to their audience. If a broadcaster starts screwing up consistently -- for example, by relaying false or unsubstantiated information -- the audience has the choice to move on to the next guy.
I also don't agree that it's necessarily the platform's responsibility to control what's on it, although it has that option.