specific rule. Since the 1960s, it's been illegal to steal signs using a mechanical device or technology. It's a specific rule that is widely known by every player and coach.

If you can steal a sign using your naked eye while sitting in the dugout or standing on the field, then that's perfectly acceptable.

The reason for the difference is teams have over a hundred years of practice relaying signs in a way designed not to be stolen by the human eye. Everyone is on a level playing field. No one team has any significant advantage over the others.

If you allow technology, some teams will have an advantage over other teams. The home team would presumably always have a significant advantage over the visiting team. Teams with better technology personnel will do a better job of stealing signs than teams with lousy tech people. Baseball will turn into a technology arms race with teams trying to find the newest and latest way to steal a sign or foil other teams stealing their signs. It will create an imbalance where bad teams can regularly beat good teams. It shifts the competitive factor in the game from player performance to technology.