Are private message systems in apps generally governed by this or is there something about facebook that makes this a special situation?
In this case, Facebook is refusing to let you submit a form, just like when you omit a required field. If you want to talk "wiretapping" or "intercepting", what about corporate spam filters and false positives? That is literally sitting between you and a legitimate sender and is throwing away data some algorithm doesn't want you to see. Sounds like this guy's definition of "wiretapping" to me, but we all know that email would be dead without spam filters.
Anyway, this is an issue for facebook and its users. If its users are willing to be censored in exchange for being able to share low-res pictures of their friends smoking pot, then so be it, that is what their customers want.