REPLY: Correct...and consider this. Team A does better by fumbling the ball. If he had held onto it, the foul would have occurred during a running play and the penalty would be enforced from the end of the run.
[Edited to clarify: One thing that's important in the original play WHERE the did the ball actually become loose. If it became loose behind the NZ then everything said thus far is true. If it became loose beyond the NZ (say, A's 30) and then rolled back behind the NZ, enforcement would be from the end of the rune--A's 30.]