Maybe if you call a journal a "writer's notebook" instead, it becomes a source of material rather than mere documentation of one's life. But isn't there a distinction between a notebook and a journal? A notebook would aim to provide a ready source of material, a journal would aim to document moments or feelings, no thought for material purpose.
I suppose a good, or at least thorough, writer would keep both—the notebook aimed toward public viewing, the journal held more private.
What brings up this observation is a long piece on David Sedaris in the Washington Post Style section a few days ago. In it, the reporter, Monica Hesse, mentions that Sedaris keeps a small, spiral-bound notebook in his shirt pocket, in which he records observations of everyday life, with the intention of spinning them into stories. Hesse observes:
If your life, however, is writing about your life, then how do you find time to live in ways worth writing about? Does being a famous self-parodist make it harder to be a good self-parodist?
I don't want to end up as a caricature in one of Sedaris's stories, so I don't relish the idea of ever speaking to him. Surely, though, there are people with the opposite desire, who want to be granted some kind of immortality through his textual alchemy. Wouldn't this affect how they interact with him?
Maybe that's true for any writer. An acquaintance once told me that she had a friend who attended a dinner party with Joyce Carol Oates. The friend related an interesting experience and a little while later, like clockwork, the story had been turned into a piece of fiction by Oates. It was no longer her story.
Best to keep your lips pursed around those super-absorbent writers if you don't want your stories taken. Perhaps, though, such super-absorbency is the mark of a great writer, and should be the aim for any journal writer or note taker, to take what you need and make with it what you can.