Is the info being provided by the user, or is the app detecting info that is no longer being provided by the user, i.e. lack of friend status is not the same as a notification that you are no longer friends. If the deal is with regard to the former, I can easily imagine that FB doesn't tell you these things intentionally. If they leave it as a silent disconnect, they can construct their TOU to prohibit detecting connection state since FB does not actually provide that data. I guess in other words, FB may simply prohibit extracting any information from their service that they do not explicitly provide.
I'd speculate that extracting more information here is a gray area in general but it should be clear that this would be the "blackest part" of the gray - ie, potential stalking/exploitation material.
Facebook lets people post where they are when. An app that determines when someone isn't home or when someone is alone in a dangerous area would still be undesirable.
Note also - Friend doesn't let someone explicit reject a friend request and it doesn't let people "unlike". This may seem dictatorial but in many ways it's "good moderation/curation of the space". It's just as logical for Facebook to remove apps that want to add "the forgotten" "reject button"/"unlike button".
I doubt it's a gray area in the FB offices. The strategy is about more info in than info out, so the flow directions are certainly controlled resources. The rules don't even allow you to use someone's FB profile picture unless they're currently logged in. This is an important point.
