They could leak our story to get their point of view spun. I do not like surprising people. Sometimes, though, when you're investigating, you want to do the tough interview last. Reporters use various techniques to check on their material. Others read, or send faxes of, quotes or parts of stories for verification or comment.
Reporters dread checking a great quote with a source for fear of the source's backing away from it.
But if they feel confident it is accurate they go with it. Editors also worry about pre-publication disclosure of stories because it may increase their exposure in a libel suit. Say a reporter shows a story to a subject, and the subject says it's wrong, yet the reporter relies on other sources and goes with the original story. If the story does turn out to be wrong, the damaged subject may claim that the reporter showed a knowing disregard for the truth, thereby harming the paper's interest. We ought to let them know so they can tell us what we need to know to balance the story.