Is a stranger's laptop worth getting your face smashed for? Or stabbed? It doesn't sound like anybody's physical well being was threatened, so why risk your own?
Because violent street thieves are unable to walk into a coffee shop?
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.
Have you been to San Francisco? I don't know the rate of stabbings, but I have seen plenty of violently unstable people here that I wouldn't want to provoke.
I agree that it's quite inexplicable why a woman who didn't know the owner of the laptop ran after the thief who was at least a third bigger than she was.
I didn't actually see him, but he was a grown man by all accounts, and the woman was on the smaller side. So she was something like 120 lbs. max and he was at least 180 lbs.+