It is just another in a long line of colloquial phrases that have cropped up in the online gaming industry. It is akin to phrases like "mob" or "agro" or any other number of catchy little terms used to express something in a game.
I have seen a couple people get into a fit over the use of the word "toon" and what I want to know is why it seems nearly exclusive to this term. I rarely see people making hate filled posts about the use of the term "mob" in games. It all seems to be directed at the word "toon".
I wonder why that is...
In any event it is just a term that people use to simplify life. If you must draw a comparison... why do you call a twenty-sided die, a "d20?" Why all the subterfuge? Why not just call it a twenty-sided die?