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 throughout the process of contacting 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.
What rule gives us the act of catching a pass?
RULE 8 FORWARD PASS, BACKWARD PASS, FUMBLE
SECTION 1 FORWARD PASS
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; and
c) maintains control of the ball long enough, after (a) and (b) have been fulfilled, to enable him to perform any act common to the game (i.e., maintaining control long enough to pitch it, pass it, advance with it, or avoid or ward off an opponent, etc.).
Note 1: It is not necessary that he commit such an act, provided that he maintains control of the ball long enough to do so.
Note 2: If a player has control of the ball, a slight movement of the ball will not be considered a loss of possession. He must lose control of the ball in order to rule that there has been a loss of possession. If the player loses the ball while simultaneously touching both feet or any part of his body to the ground, it is not a catch.
Item 1 is talking about a receiver going to the ground.
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
A player at this point is still a receiver and must meet the Item 1 requirements.
When a player does this:
c) maintains control of the ball long enough, after (a) and (b) have been fulfilled, to enable him to perform any act common to the game (i.e., maintaining control long enough to pitch it, pass it, advance with it, or avoid or ward off an opponent, etc.).
They become a runner and Item 1 no longer applies to that player.
And that is EXACTLY what those case plays illustrate.