With no way to log off, that leaves a session hanging out there, which somone else could intercept.
As it is now, killing a session within your browser doesn't actually terminate the dropzone.com session. Because if you re-activate dropzone.com, you jump right back in to your previous active session.
The only way to actually kill the session is to kill the web browser, and then re-boot it to continue your work. This interferes with other work going on at the same time.
As Bolas mentioned, you can log out on any of those pages. The other 10 login/logout boxes should work fine on all the other sections, and they should all be global. The 'my stuff' bar on the forum, assuming one has it showing, offers a log out option. Otherwise the other pages (dropzone, gear, photos etc) offer a logout button on the top right next to help.