Of course they shouldn’t.
But, they’re private entities and one agrees to their Terms of Service when logging on. If they want to decide who uses their site and who doesn’t, that’s their business.
It’s not as if users wouldn’t possess a block button. I can’t recall a single time, in my decade using social media, that I opened an Infowars link. I doubt most of you have, either. Even if you did, you wouldn’t melt. They’re just words.
And the very last thing we need is some bureaucrat or politician telling them what to do.
More @ Social Media Giants Shouldn't Be Arbiters of Appropriate Speech