I think I'll need to preface this by saying that Johns shouldn't have said what he said, however...let's try to understand why things may have happened the way they did...
I think, and everyone has been there, when you're in a vicious argument with someone, you pull out what you regard to be the most insulting remark you can think of (usually referring to their physical characteristics) and you then use it as a verbal bullet. I know, growing up as a kid, when I had arguments with my sister, I used to refer to her weight and use that as a verbal bullet.
I think if we can get into Johns' mind, when trying to rev up his teammate Beau Scott, he tried to use the most vivid and insulting remark to attempt to get his point across forcefully and impress it upon Scott. Unfortunately, that involved Inglis's race, however if Inglis was 200kg and had freckles, I'm sure the description would have been "shut down that ugly, fat c*^#%".
I guess that explains why Johns has only been heard to use these racial insults in team meetings etc, that is to fire up and appeal to his teammates by picking out the most obvious and what he thinks is the most insulting physical characteristic of a non-white player.