I think it was a poor decision by the officials. He was in full control of the ball. What he was not in control of was his momentum and to avoid falling face first, he decided to use the hand holding the ball.
He clearly put it down. It's the same as if he made a catch on the boundary but then had to step over the boundary to keep his balance. It doesn't matter if he's in control of the ball, you can't put it on the ground for balance.