You misunderstand, becker. If the girl wants out, then yes, it's in your best interest for her to back out one way or the other.
But especially leading up to first dates, there's a lot of anxiety, and we often try to relieve anxiety by removing the cause of it (in this case, by cancelling the date). Most girls won't just call you up especially to cancel a date, but if you call her up to confirm, you're essentially saying, "Are you sure you want to go out with me?" And you might get a cancellation you wouldn't otherwise have gotten.
Once again, there is no reason to confirm something that's already been confirmed; it makes you look unsure of yourself. If someone asks you to do something, and you agree, and then they later ask, "Are you sure?", the first thing most people do is wonder if maybe there isn't a reason they SHOULDN'T cancel.
Look, you put yourself on the line and risked rejection when you asked for the date. She said yes. Why ask again? That's a bet you can't win.
Think of it this way... would you play against me in a game of poker where I tell you to put $100 down... and if you win the hand, I'll give you your money back, but if you lose, I get your money? That's basically what you're doing when you call to confirm.