Force Out Ruling

Question
Here's the situation from a recent game.  Runners on 1st base and 2nd base, no outs.  Batter hits a low pitch which the runner on first thinks is a line drive, but it is actually a ground ball, having hit the dirt from the bat right in front of the plate.  Pitcher fields the ball, and throws to first to get the batter/runner in plenty of time.  Runner on first base never leaves the bag but is not tagged.  Runner on 2nd base is off the bag, sees the first baseman throwing to second, and gets back after the shortstop catches the ball on second, but is not tagged.  Ruling was that batter/runner is out but runners on 1st and 2nd are safe.  I know the throw to first takes off the force at 2nd and 3rd, but doesn't the runner at first base have to leave the bag and if so, wasn't she out when the throw was made to first along with the batter/runner?

Answer
Hi Vince,

before a play was made at 1st R2 was no longer legally entitled to it but there is no rule that they must leave the base.  If tagged while on base they would be out.  

However this all changes when the b-r is first put out at 1st, now all forces are off and R2 can once again legally occupy 1st.  Had F3 first tagged R2 on base and then contacted the base you would have 2 outs.

The ruling on the field is correct.

mark