There are several reasons why seat backs need to be upright for take off and landing… some of those are to protect the occupant of the seat, and others are to protect those sitting around the seat.
If there’s no one sitting around the seat, and the occupier of the seat is competent and sufficiently informed to accept the risk, why should I worry? I’m just not sure I’d be as exercised about that particular example as Mr Cahill.
The tablet didn’t look that much bigger than what we’re allowed to hold in our hands in Australia for takeoff. Which is a bit silly because in the event we’d need to assume the brace position instantly, where do those hand-held devices end up anyway? Loose in the cabin!
Something might be a technical breach of the rules, but if there’s no impact of that, what is Mr Cahill’s concern about? Some airlines don’t turn off the seatbelt sign for the whole flight, despite smooth flying. But people still need to go to the lavatory. Do we have a video on that?