Booksellers are tied to publishing - they need conventional publishing models to continue - but for those companies, that's not the case. Amazon is an infrastructure company; Apple sells hardware; Google is really an advertising company. You can't afford as a publisher to have those companies control your route to market.
I'm not an absolutist about free speech. Intellectually, I believe that most of the time it's better to let things get said, argue them, and put lies and stupidities to rest. Practically, I know that newspapers rarely issue corrections with the same prominence they give to denouncements - and Twitter, by its nature, never does.