Not fact checking is only picking a side once you have decided to fact check. Before all of this twitter could claim to be a medium of communication. There isn't implicit trust that something is true because it is written on paper, posted on a billboard, or appears on TV. By choosing to mark tweets as true or false, they are no longer just a medium of communication. We can now imply that if twitter does not mark a tweet as false, they are endorsing that tweet as true.