Ah!

So we're relying on the cashier to care deeply enough to do this validation.

Remember the phase when credit cards were being issued with a picture of the card holders face on the front? And all the stories about the card with a picture of a petite Indian female being used by a large white male to ring up purchases?

I like the concept of Square, and I'd like for this to work. But the cashier being a face-recognition gateway seems like a horrible idea.

Yep, you can't enable the tab feature without first taking a photo of yourself with the app.

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.

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.