Because the obstruction occurred prior to the runner going back to third. The runner did not stop at third, he returned to third. The umpires judged that the BR would have made third without the obstruction, so that is what the rules say to award him.
The official pro interp (on Type B Obstruction) is that the umpires must be absolutely positive that the obstructed runner would have made the next base, otherwise he gets his return base. It sounded as though the umpires were certain that the BR would have easily made third, so that's where they put him.
Jim Evans says: "When Type B Obstruction occurs, the umpire must make an "initial decision" to which base he will protect the runner. That is based on the position of the runner, the speed of the runner, the position of the fielder, and the location of the ball at the very instant the obstruction occurs.
That "initial decision" may change based on subsequent events; e.g., ball eludes a fielder or ball is dropped by a fielder."
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Matthew 15:14, 1 Corinthians 1:23-25
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