Because the parents have reached a point where they can no longer pretend that little Johnny is telling the truth when he says it's no big deal, and he's doing better now.
My favorite "can't he just do a report?" story:
Toward the end of last year, a student asked for emergency extra credit, and I did, actually, for various bad reasons, assign him a report, on a historical topic of his choice.
He turned in a very long, very hard to read report about Christopher Columbus. Assuming that this kid, bright as he was, had not written the words 'from his humble origins in the thriving Renaissance town of Genoa'...it was something like that...I Googled a few phrases, and found the original article.
I told him that I wasn't even going to kick his butt for plagiarizing. Worse. I was going to insist he rewrite the thing in his own words.
This ended up with a parent conference in which the parent genuinely did not understand why this was a problem, and eventually uttered the now famous (at least in my family) line--"In this day and age, can you really expect a thirteen-year-old to have anything original to say about Christopher Columbus?"
Fade to black.