The NRL hasn't been enforcing a lot of rules for a long time, this is one of them, which is probably the issue.
You can't just obstruct a defender from getting to the ball unless you're also competing for the ball. 99% of the time the player goes around the player who is obstructing them. I'm surprised more players don't do what Wighton did to be honest.
The defender isn't expected to disappear, and can stand his ground, he can't move into the path of the chase to obstruct them, but neither does he have to get out of their way, so absolutely a player can get in the way of the chase.
He just has to get there early enough so as not to be deemed as moving into the path of the opposition player.
For example, two players from the defence can be going for the ball, one player calls the other out of the play, so he is no longer going for the ball, your interpretation there would mean he is then obliged to move out of the way of the opposition players attacking the kick, which clearly is not the case.
In the case of JA's penalty, whether it's a legitimate penalty or not relies on whether he's deemed to have deliberately moved into the path of the Wighton, not whether where he happens to be is in Wightons way.
Obviously at some point he ( JA ) moves into that position where he ends up in the way, penalty or not is decided upon how early he gets into that position, because that's how they judge his intent.