Richard J. Anderson makes an excellent point:

Freedom of speech is not, and has never been the same as freedom to be heard. Censorship is an organized effort of a governing body to silence something they don’t like, and it can be done by a government or a corporation alike. Individuals choosing who they do not wish to hear, and collaboratively tuning them out is within their rights by any legal framework. If Twitter, or any other service, were bound to force users to see replies from any Tom, Dick, or Harassing Harry on their service, it would be akin to Fox News having the legal right to pre-empt you watching CNN on your TV. (And that analogy works if you flip the networks around.)