Those super-rare cases are something the press can write about. Nobody would read a headline titled "A 40-year old man spent $500 on virtual in-game stuff". Now, just replace 40YO man with a 10YO kid and you have a story.

The mere fact that we aren't swamped with such stories (I can recall reading maybe 4-5 over the past 5 years) confirms that it's really, really rare thing to happen.