Dropbox has been luckily spared of any major copyright infringement scandal and the copyright-dependent industries haven't targeted it, yet. But it's treading a fine line. On the one hand, it keeps copies of whatever users have in their Dropbox folder, legal or otherwise.
"Worse" than that, it enables users to share anything they want publicly. The company does make it clear that you're not supposed to share infringing material, but there's no account of it ever blocking any link, until now. Users have found that they can't share the now infamous Xbox 720 document publicly.
Existing links to the file now point to a warning indicating that the content has been removed due to a complaint by Microsoft.
Dropbox is not messing with your files, you can still upload the document to the site and even share it with other users. But you can't share it publicly.