Bottom of the 7th, 2 outs, runners on first and second bases, game tied 13-13. Sharp 1-hopper hit to the hole that the shortstop dives for and snags. The runner on second base mistakenly thinks that there is no runner on first base and turns to return to second base. After the base coaches and dugout yell loud enough, he breaks for third as the SS makes the easy flip to the third-baseman. He tosses the ball over his head though, and it goes into the dugout. Ump rules the runner scores on the ball being thrown out of play. Guy in the stands contends that he should be out for going back towards second base. Besides the fact that the baserunner should get kicked in the nuts, what's the correct ruling on the play?