Here's the text of the rule along with some of POD's commentary.
Article 3. Completed or Intercepted Pass. A player who makes a catch may advance the ball. A forward pass is complete (by the offense) or intercepted (by the defense) if a player, who is inbounds:
(a) secures control of the ball in his hands or arms prior to the ball touching the ground; and
(b) touches the ground inbounds with both feet or with any part of his body other than his hands.
. . .
If the player loses the ball while simultaneously touching both feet or any part of his body other than his hands to the ground, or if there is any doubt that the acts were simultaneous, it is not a catch.
Based on that part of the rule, Johnson made a catch. The caveat is the "Item 1" portion of the rule, which deals with going to the ground.
Item 1: Player Going to the Ground. If a player goes to the ground in the act of catching a pass (with or without contact by an opponent), he must maintain control of the ball after he touches the ground, whether in the field of play or the end zone. If he loses control of the ball, and the ball touches the ground before he regains control, the pass is incomplete. If he regains control prior to the ball touching the ground, the pass is complete.
This specific item is the main problem with the rule, because there is nothing specific about how long a player must maintain control of the ball after he touches the ground. It simply states that he must maintain control after touching the ground, which is where the idea of this being a process comes from. The problem is there is nothing about how long of a process this is, which leaves the rule open to complete interpretation. While some, including the officials, may see the play and think it was incomplete, others, like myself, believe it was a catch.
And here's the conflicting "Item"
Item 3: End Zone Catches. If a player catches the ball while in the end zone, both feet must be completely on the ground before losing possession, or the pass is incomplete.
So, reading literally, Johnson satisfied 3 a and b. The POD post suggests that Item 1 implies he did not make full contact with the ground before he placed the ball down. However, that reading seems to assume that we cannot count his two feet and butt making contact as "touching the ground". Why the hell not? With that natural reading there is no conflict between parts of the rule. So, it looks like a bad interpretation of the rule, not a bum rule is to blame for this fiasco. Indeed, Item 3 reinforces this reading.