Look and listen. That tells all.

Patrick Lambie of the Sharks charges ahead. Schalk Burger of the Stormers engulfs him. They go to ground.

The referee tells Burger to roll away and then penalises him for not rolling away. When the thing disentangles it is clear that Burger has not rolled away.

This is what the law says about the tackler.

Law 15.4 THE TACKLER
(a) When a player tackles an opponent and they both go to ground, the tackler must immediately release the tackled player.
Sanction: Penalty kick
(b) The tackler must immediately get up or move away from the tackled player and from the ball at once.
Sanction: Penalty kick
(c) The tackler must get up before playing the ball and then may play the ball from any direction.
Sanction: Penalty kick

No wonder Burger did not argue.