It is indeed rude for Facebook to scan private messages without our knowledge because these are private. While on the other hand, Facebook move was to keep everyone safe away from child exploitation imagery and viruses - as mentioned above.
So what do you think? Does Facebook's move good for all? Did they revised their terms of service?
Facebook as of now is doing all the measures to eliminate privacy concerns as much as possible. But these things are bound to happen on the internet. Even Google is watching your every move and you might not know how much data they have on you. So ff you are too afraid about your privacy don't use the internet at all.