The reason that they can be Googled is that Google Toolbar may be configured to pass URLs that one visits to Google for indexing - if you run Google Toolbar, it knows pages you visit.
So if this is correct, Google in its wisdom has decided to treat a URL sent to someone with the Google toolbar in a private email as a publicly reachable URL.
If this is correct then I'd be stripping off the Google toolbar (a completely separate plugin - not the same as Firefox's inbuilt search bar) as quick as I could.