In my room, we have 16 tables and they are in close quarters. Our rule is that players in cash games may have a player sitting behind them as long as they do no communicate while the player has cards. In tournaments however, players must remain "a table length" away, which we enforce as "players may be no closer than the edge of the nearest table."
Our reasoning is, if a player in a cash game doesn't like a spectator sitting close to them, they have the option to change seats or tables, but a player in a tournament who doesn't like it wouldn't have that option available to them. Also, because players in a tournament are generally under more pressure, especially near the money bubble, and we want to limit the distractions to those players.