Update (1/15): If it’s not wrong for players to steal signs (and not a single baseball player on the Houston Astros was punished by the Commissioner of Major League Baseball), why is it wrong for team managers or the front office to steal signs?
According to Wikipedia, sign stealing in baseball has a long and venerable history: the oldest recorded instance of a baseball team attempting to steal signs dates back to 1876! Furthermore, it turns out that sign-stealing in itself is not wrongful, but rather what matters (in terms of moral blameworthiness in baseball) is what types of signs are stolen, by whom they are stolen, and the ways in which they are stolen. See, for example, the following passage from Wikipedia (footnotes omitted):
According to the unwritten rules of baseball, stealing the signs that are given by the third base coach, or those of the catcher by a baserunner on second base, is acceptable, and it is up to the team giving the signs to protect them so they are not stolen. However, a batter peeking in to see the catcher’s signs is considered a violation. The signs the catcher sends to the pitcher to call for the next pitch are considered more “sacred” than the signs a third base coach relays to the batter.