CR/SFG Catcher or Batter Interference?

[SAump]

Situation from photo on Yahoo Sports:
"Home plate umpire Dan Iassogna, second from right, throws out Colorado Rockies manager Clint Hurdle (13) as Hurdle came out to argue a batter's interference-out on the Rockies' Clint Barnes, at left, against the San Francisco Giants in the sixth inning of a baseball game on Saturday, Aug. 5, 2006, in San Francisco."

The dispute came as a result of a pitch out. The batter reached across the plate and swung at the pitch striking the catcher's mitt. The swing was terrible and probably made to delay the catcher's response. The batter may have hit the ball had the catcher not been there. A judgement call had to be made one way or the other, one I believe could go either way. I am interested if anyone has any baseball rule, custom or tradition to help an umpire confused by this particular situation.