They know Mary Jane ain’t going to kill them. Lying about it, pretending it might just proves to them you’re a liar or a fool and that nothing you say is worth taking seriously.
Wouldn’t it be better to just tell the truth?
“What will your mother say when you get out of the slammer and she discovers you’ve been anally raped by every ethnic prison gang ranging from the White Brotherhood to the Crips, the Bloods, to La Raza Unita?
“That you were forced to perform oral sex every night for the guy in the top bunk?”
Mommy wants that to happen to OTHER peoples’ kids and doesn’t want you to know she wants it for them, but she sure as hell doesn’t want it to happen to hers. So she tells you the next best thing. A lie.