[mbyron]

If Selig had decided to overturn the call, I'm sure that the justification would have turned on claims about fairness and getting it right.

He wouldn't have to go through the players' union or WUA because it would have been an exceptional instance and he wasn't proposing any change of policy or rule.

As for whether he has the "power" to overturn an umpire's call: that's moot. The powers of the commissioner are ill-defined and broad. I suspect that had he decided to overturn the call most umpires and many others who love the game would have regarded it as an illegitimate extension of the commissioner's power.