Because the man does not have to let him up.....he must allow the player on the ground to do one of three things, but does not have to let him up....this was discussed in earlier forums.
Actually, JB I do believe you have the wrong emphasis even now.
Man on floor with ball, man on feet wanting ball. Law 14 Definition 'The game is to be played by players on their feet... A player who makes the ball unplayable, or who obstructs the opposing team by falling down, is negating the purpose and spirit of the game and MUST BE PENALISED [my capitals].'
Man on floor 'MUST IMMEDIATELY' [mine again] do one of three things...; he does not usually have the time, nor the permission of the referee, nor the indulgence of the player on his feet who is waiting to play the ball to choose which one but he must do one of the three without making the ball unplayable. The player on his feet is fully entitled to the ball which the man on the floor may not make unplayable nor may he obstruct the player on his feet from access to it.
The ONLY, only thing the player on his feet must do is to remain on his feet, that is all.
Your mindset must be in support of the attacking player demanding the ball, providing that he remains on his feet of course. Ask yourself has the player on the floor done enough so as not to make the ball unplayable. In my book he has but a nano second to play it or get away from it so that the player on his feet's rights to the ball are not impinged.
Protecting the ball and screaming 'he has to let me up' while delaying in order for his own side to support cuts no ice here. It has the potential of a PT if close enough to the goal line or even on halfway as I saw once in the World Sevens Series.