Is it illegal to tackle a player in a ruck if they are joining it from the side?

Is it illegal to tackle a DEFENDING player joining the ruck from the side to take the ball (isn't it illegal to join it from the side too?). For e.g. In the Tri Nations I saw Schalk Burger (defending) join a ruck from the side to try to take the ball, and then McCaw comes running up and tackles him from the side too. As a result, McCaw was penalized. Is what he did really illegal?