Send back the runners on obstruction?

Question
ASA co-ed slow pitch softball. Runners on 1st & 2nd. Base hit to center field. Runner on 2nd rounds 3rd and heads home without hesitation. Runner on 1st initially holds at second because the relay throw goes to the short stop about 10 feet behind 2nd. (Batter-runner rounds 1st.) The short stop throws home where the catcher is still behind home plate and the pitcher is covering home plate. After the runner (originally on 2nd) crossed the plate safely, the errant throw hit the backstop. The other two runners advanced to 3rd and 2nd respectively. The umpire then said, "Blocking the base without the ball" and declared the runner who crossed the base safe and sent the runner on 3rd back to 2nd and batter-runner on 2nd back to 1st.

Is "Blocking the base without the ball" a rule? We equated the call with Obstruction. Although the pitcher was in the base path without the ball, there was no contact between the runner and pitcher as the runner scored before the throw would have reached home. Was sending the runners back who originally advanced to 3rd and 2nd on a ball within the playing field the correct call? If so, why would a play on which the defense committed a violation result in a penalty to the offense?

Based on this information (and if you need more details, please let me know) how would you rule? Would you send the runners who advanced on the throw back to their previous bases?

Thank you!

Answer
Hi Ken,

"Is "Blocking the base without the ball" a rule?"  Yes it's called obstruction

" there was no contact between the runner and pitcher as the runner scored before the throw would have reached home."  contact is not necessary for obstruction

"Was sending the runners back who originally advanced to 3rd and 2nd on a ball within the playing field the correct call? If so, why would a play on which the defense committed a violation result in a penalty to the offense?"  this makes absolutely no sense.  There is no such penalty.

With obstruction (if you had it) R1 was obstructed at HP.  Since they safely scored the obstruction is cancelled.  R2 and B-r3 were not effected by the obstruction and it has no effect on them.  They should have been left where they were.  By sending them back the "P" word should have been used.

Mark