When batting out of my crease if the ball goes through I'll always look to get back. Always. Even to a pace bowler. To the point its habit. That dismissal is nothing but assumption from the bat that he's fine standing out of his ground.
It's great cricket from Paine. Sloppy from the batsman. If Paine was standing up, he'd get back, if he wasn't batting in his crease already.
How is it different to the Mankad? Maybe because its happening while play is 'live.' If he'd hit to a close fielder, he'd be in his crease, why is the keeper any different?