This is by design. Basically, the draft option is just a shortcut so one doesn't have to uncheck all of the other checkboxes for "hiding" a page. It wasn't meant to secure the page, just hide it from people who don't have the link.
I'm sure there is a less ambiguous term for this functionality. We'll give it some thought.
As a side note, we're working on a lot of changes to the Page Settings page to make things easier to understand and find. If you think of a better name for this option, let me know.