This is what I do. I've personally witnessed a laptop being stolen from a cafe in SF, but only from the very front very close to the door, and the guy had walked to the back to go to the bathroom.
But at any rate, it is better to shut up and give description to the owner.
If you live in a city where violent crime is not uncommon, don't assume that a thief is a nice person just because you've caught them in the act in a coffee shop.