[nimrody]

In that case, the cashier can easily charge you without your consent. Of course, you get the confirmation mail and can cancel the transaction but this is inconvenient.

Making payments too easy makes it easier on the bad guys as well.

[stbtrax]

Yes, but the kinds of purchases this is used for is usually < $10. It's not really worth anyone's time to follow someone around to get a free coffee.