I have always found it odd that a player from their back to sit up to get a range of mobility to pass, see imminent contact/player pressure coming, and generally be in a more stable and comfortable position, yet the player on the floor on their stomach can never really get a similar position without travelling.
It would seem to me that most hustle plays would have people diving face and chest first toward the ball and floor, rather than back first
Back first tends to be a result of a broken play where someone ends up off balance or already on the floor and flailing after a ball.
Not really a reward or punishment, but if I clumsily fall down on my back side and have to fall back to keep control of the ball (because I'm also clumsy) I can sit up. If I put my body on the line for a loose ball and floor burn my knees, hips and chest to get possesion of a ball I now get to lay perfectly still in a nearly impossible position to do much from.
My biggest concerns with these plays is player safety. When a player is sitting up almost no one dives on them or at the ball. When a player is prone on their belly there is almost always someone diving in or on second to try to tie the ball up. (IME)
*As officials I think its important that if contact happens to a player laying on the floor or diving at the ball I think its important that we get a foul call. Not just call for the tie up to reward the hustle on both sides.