Batter's Swing and Follow Through

[SAump]

RULING 20: With R1 on first base, the right-handed batter B2 swings hard and misses the pitch. The catcher, seeing R1 slow in returning to first, attempts to pick him off. B2's follow-through by the bat hits the catcher and causes his throw to sail into right field. RULING: The ball is dead and the B2 is declared out for batter interference. R1 is returned to first base. A batter is responsible for the follow-through of a bat when he swings. (7-3-5c)

I read the old rule as making any other movement which hinders and incorrectly believed the swing and follow through were protected by rule. Not any more. Any idea why FED is splitting hairs once again?