The human brain is stupid and remarkably exploitable. Just a teensy little bit of information hiding can illicit strange and self-destructive behavior from people.
You aren't cut off until you're cut off, then it's over completely. That's scary, because there's no recourse. So people are going to try to avoid that as much as possible. Since they don't know how much they're using, they're naturally going to err on the side of caution - paying for more than they need.
