I think the issue here is the tackle motion. I have never understood why players, after having laid a good tackle, need to sling the player to the ground in a second motion. The idea of the tackle is to render your opponent incapable of disposing of the football, there is no necessity to throw him to the ground.
Unfortunately for Rioli, there was a second motion, it was a sling tackle and he must be found guilty. Whether it deserves matches or not is debatable.
It is happening because the umpires are not making a decision. The player is tackled, can't dispose of the ball so he is either holding it or its a ball up. The umps are waiting and waiting for the player to get the ball moving so the tackler is feeling like he needs to do more and hence starts slinging the guy to the ground. If the ump had of just said he will ball it up straight away in the Rioli case nothing would of occurred.