The argument for Twitter is that they don't want to be forced to be one or the other. They want the protections of the former, with the editorial privileges of the latter.
To be clear, I don't agree with them, but I can see why they're aiming for a grey area in a hyper partisan environment. They need to keep their progressive users retweeting in outrage while ensuring their conservative posters keep shouting from the rooftops. It's a fine balance they manage to keep everybody tweeting.