I loved it from a NSW side, but you cannot do that when you are not on the field.
The problem is that the game is guilty of trying to officiate at the elite level in the same way they do at lower levels. This is why players get suspended for accidentally running into a ref; they are so scared of setting a precedent that would potentially be followed in lower-grade leagues.
If they did not ban him, people might think that it is an okay thing to do (as stupid as that is). Also, there is a fair difference between an assistant who was there pulling players apart and a 19th player running from a distance to get involved.