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?
Ah baby! Yes. Yes. YES!
“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.
So Far From Heaven 
Mom told me (lied to me) early in my life not to lie because my nose would grow. Being the kid I was, I tried a few lies just to gain a few inches of nose length, then I would quit. Found out that stretching the truth would not make your nose grow. I learned later on in life that the dangers and punishment for stretching the truth was much more severe than that of nose growth. Just wish she would have told me the truth and saved me some of the agony I have suffered beyond having a real honking nose. I have been mighty careful most of my life to “not do” what my Mom told me not to do.. well mostly. I do wear glasses.
Hi Eddie. Bifocles? J