It's not simply trying to figure out the opposing teams signals during the game. It's using electronics and video equipment to do it live and then signalling to the batter what pitch is coming.

I know it doesn't seem like a big difference, but it gives the home team a significant advantage. The away team can't very well post up cameras in the outfield, and connect them to a tv screen in the dugout tunnel, but for the home team, that's not that difficult to make happen, which gives them an unfair advantage.