The NRL went to specific lengths to change that rule because players were doing it all the time in the late 90's. Every time a kick was put ingoal, the fullback would put one foot over the dead ball line then reach in and bat the ball dead.
I hated that, and was glad they stopped it.
The theory was as the player was out of play, he formed part of the out of bounds area. So when he touched it, it wasn't him touching it, it was "out of bounds" touching it. So it was deemed not to have been touched by the defender. Which lead to the farcical situation of that in goal nonsense, where the defender was clearly making it dead, but ruled not to have.
Problem is, this concept still applies, just not in that way, with the one foot in, one foot out tactic. The ball should be out of bounds when it touches the ground out of bounds, or when it touches a player who is out of bounds, with the toucher deemed to have put it out. That would rule out the straddling the line trick, but clean up the rule properly.