[jjmardlin]

Despite the comments that the slogan predates advertising, there is something to the parent commenter's thesis, though it's certainly not pure cause and effect.

This is a commonly discussed psychological phenomenon; if you focus on avoiding the negative, you accentuate that. You're not saying "Be Good", it's don't be evil, and so you not only name the elephant in the room, you chart the path around her periphery, and may actually crash into the elephant of evil.

In other words, as taken from a parenting site:

State Requests Positively

>Positive requests are more effective than negative commands. For >example, your child is more likely to respond to, "Please use your >spoon." than "Don't eat with your fingers."