Part of the problem here is that the privacy users care about here is not from a third party, but from whomever the recipient might otherwise choose to share the content with.

And that pretty much requires a party other than the recipient whose interest it is to lock down the client as much as possible to at least increase the hassle-factor in violating the senders trust.

Otherwise, if the recipient had the "EvilReplicatingToFacebook" fork of the app readily available, it'd quickly destroy any value in the app.