I don't get it

[tcannizzo]

Play: With two outs and R1 on 2B, B4 hits a ground ball to F6. F6 fields the ball and throws high to 1B. F3 jumps up and catches F6’s throw and the throw takes F3 onto the contrasting color of the base. F3 only touches the contrasting color of the base before B4 reaches 1B. Is B4 out?
Ruling: Since the throw from F6 was an errant throw, that in the opinion of the umpire took F3 to the contrasting color of the base, the batter-runner would be out. (Rule 8, Section 2M [5]:On an errant throw pulling the defense off the white portion of the base into foul ground, the defense and the batter-runner can use either the white or contrasting portion. If the umpire judged the throw took F3 to the contrasting color portion of the base that is in foul ground, then F3 can use either portion of the base. Note the throw does not have to take F3 beyond the contrasting color of the base to be considered to have pulled the defense into foul ground.)
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Why is the defense rewarded for an error?