CYaBro: It does say in the rules though that a penalty or yellow card should be given, not a red card.
Quote 10.4k, IRB Rugby Union Laws
(j) Lifting a player from the ground and dropping or driving that player into the ground whilst that player's feet are still off the ground such that the player's head and/or upper body come into contact with the ground is dangerous play.
Sanction: Penalty kick
The rule was amended in 2010 by the IRB, taking a zero tolerance stand on spear tackles, which makes the red card the only option.