As much as we would like to believe that our parents are the epitome of morality and justice (LOL no one actually thinks that right?), at the end of the day, they're just candy-loving monsters in grown-up skinsuits who like, pretend they know how to do taxes but then get John from down the street to file their T4 for them.
What I mean is that kids and adults are basically on an equal playing field when it comes to impulse control, so when a holiday comes along that offers us an unlimited supply of sweet treats, why wouldn't parents want to get in on it?