I'm not sold on the fan voting either.
With that said, fans do, at least, have the benefit of instant replays.
Remember the reaction to Vince's elbow dunk when you watched it on TV? It was shocking, yet the fans at the event didn't react until they saw it on replay on the big screens because they couldn't make out what was going on. The judges were similarly unimpressed from their vantage point, because they couldn't really see what happened.
Ditto Dwight's sticker dunk - it was incredible, but it didn't look like anything to the judges except a guy dunking and slapping the backboard.