Because, once it's a threat, it's too late to cap and meter. It's much more difficult to take something away from someone after they've experienced it than to deny it to them in the first place. This is a lesson that the parents of every toddler out there learn very quickly. If you don't want your kid to throw a tantrum over a toy you don't want them to have, the best thing you can do is to never let them lay eyes on it. Once they do, it's game over.