Baserunning-Options once a force is off

Question
I have a play question which may have been asked but I didn't see it  I watched a game today where there was a controversial call.  It was a little league game but they play very similar rules as major league baseball.  There was a runner on first, a fast ground ball was hit to the first baseman.  The first baseman stepped on first with the ball.  The ball was hit so fast that the runner on first dove back to first base.  With the force off because the first baseman stepped on first, is the runner on first still obligated to run to second or can he return to first base before being tagged and be safe.

As a follow up question, if the runner on first is obligated to run to second once the force is removed then what difference is there between having to tag the runner going to second and stepping on the second base bag only.  In this game, the runner at first was called out because the first baseman tagged him out while he was on the first base bag after stepping on the bag with the ball getting the hitter out.  If the runner is obligated to run to second even after the force is off what difference does it make to have a force or not.  I hope I am making sense.  Just curious which is correct.
Please help,
A confuse baseball fan

Answer
AS I understand it, the runner is safe at first.  The force was removed when the first baseman tagged the bag.  The runner is then free to reach either base safely.  The defense is obliged to tag him before he reches the bag.

I have been known to be wrong on occasion when it comes to rules....but I think this one is pretty clear.

Pete