And no, it wouldn't dictate what you could say or believe, but it can help you say it. And if you need to say something that a powerful entity doesn't want said, they can protect you from DDoS and other attacks.
It's one thing to advocate that entities not make certain kinds of speech - by all means, please advocate away! But CloudFlare is an assistant to speech, not just an advocate. The moment you start making rules and decisions about what you enable, you open yourself up to the human possibility of making wrong rules and decisions. And that means it's better to not make the rules in the first place. "Better that ten guilty persons escape than that one innocent suffer" and all that.