Is this similar to how the phone company operates? As long as they just transmit your voice to the other party they can't be prosecuted for anything you say. But, as soon as they start listening in and commenting on your words they have to listen to everyone's conversations and comment on all the words. Obviously phone companies don't want to be responsible for everyone's words so they don't comment on anyone's words.
The parallel would be Twitter commenting on a tweet. As long as they just pass the tweet on to other people, without commenting on it, then they aren't responsible for it's content. But as soon as they say "hey, this might be false information" about a specific tweet then they are responsible for the content of all tweets.
The parallel would be Twitter commenting on a tweet. As long as they just pass the tweet on to other people, without commenting on it, then they aren't responsible for it's content. But as soon as they say "hey, this might be false information" about a specific tweet then they are responsible for the content of all tweets.