It could still be a copyright violation if a third party is scraping/distributing and profiting off of something you wrote. Here's an interesting opinion on how forwarding private messages could be considered copyright infringement: http://nulawreview.org/extralegalrecent/do-not-forward-why-p...

So perhaps it's not an expectation of privacy, but more of an expectation that your private messages won't be distributed by third parties for profit.