Gibbs' one is really not a perfect tackle, it's a double-action slinging and deserves to be penalised as it's immensely dangerous. The perfect tackle is using your own weight to bring down the player you are tackling, or using existing momentum to bring them down.
The Richards one... well, Schultz hasn't yet been suspended unless they've just introduced a special lightning round for the tribunal that sat and judged as Richards was stretchered off, but it's actually a different process with the same outcome - it is an example of the tackler using his own existing momentum to bring down the other player. Very different to Gibbs.