What you're overlooking is the 'trust' factor.
I'm going to assume, never a good thing to do, that their PR people weren't aware of the editing. The reason for the photo was to alleviate the pressure they have come under regarding the lack of information in two areas. The health of Kate, leading to all manner of speculation and why William has missed events, the most important one being his absence from the memorial service for his godfather,King Constantine 11 of Greece, in which he was scheduled to deliver a eulogy or a reading. The Palace (Kensington) issued a statement that William wasn't going to attend because of "personal reasons". There was another issue re Kate,too. She is Colonel of the Irish Guards, the regiment trooping its colour this year..June 8th and,as such,she would review the regiment. Tickets were being sold stating this until Kate's representatives..ie "Kensington Palace," told them to stop and ticking them off saying that they will decide if she's attending or not, not the army.
So..in an effort to counter all the speculation re Kate, in particular, they send a happy, smiling family photo to the press agencies that has been edited or, as the more cynical would describe..manipulated. It was such an important message to get across that the sensible thing to do was to just get one of their regular pro. photographers in to get the best shot, which I see Lindsay suggested As we're all, to one degree or another, photographers, we know to 'look round' the scene, to check everything is ok. .It was also extremely naiive of Kate to think she could edit a photo that was going to be seen around the world that wasn't going to be closely scrutinised. You only have to look on here at some of critique regarding someone's efforts to edit without saying they've done this that or the other. Egg on face comes to mind..I don''t mean the basics..exposure, NR, sharpening..that's the norm but why are those steps behind the right knee of Louis not aligned ? Was he 'added' ? ...probably not but it sows doubt. Some on here have mentioned out-of season tree foliage, too. It raises all sorts of speculation..the very thing they issued the photo to quell.