From a comment further down, it sounds like it's due to considering it now redundant, and not wanting to support two ways of sharing files. The original way to share Dropbox files over the web was to drop them in your Public folder, but now you can right-click on any file, in any folder, and generate a shareable link to it. So they're going with that as the new sharing mechanism, and phasing out the "drop it in your public folder" approach.