I had a very similar situation about 6 months ago while driving on my motorbike with my girlfriend on the back of the scooter. We drove in front of the hotel on Ren Min Nan Lu, about 20 meters from the Bookworm, when we suddenly saw a guy sprinting after a bypassing high school girl. Initially I thought they were together and that he was trying to jump on the back of her bike until I noticed she was not aware of him chasing her. Still not realizing the actual situation I said to my girl “Look at that guy, he is behaving quite odd” and that was exactly the moment when the guy was catching up with the girl and was reaching for the girl’s iPod earphone cable. I immediately knew that this was not her friend but a thief trying to steal the girl’s iPod so instinctively I started to shout at him and the thief looked at me full of hatred but slightly startled so he couldn’t catch up with the girl as was still cycling further as she didn’t hear my shout nor noticed he whole situation happening behind her. I quickly accelerated to catch up with the girl and my gf pointed out the situation to her. I was looking back whether the guy was following us or not but I didn’t spot him anymore so I drove off.
Afterwards my girlfriend said exactly what Chris’ girlfriend said and that I shouldn’t do that because thieves are dangerous and carry knives and are never alone in this kind of situations.
She is originally from Guangzhou and heard several cases there where foreign guys bust thieves and immediately get stabbed by either the thief himself or one of his buddies in order to scare off any other person who has the thought of intervening.
As Ouk mentioned already, this might not be the case in Chengdu (yet) as the thieves are mostly unorganized petty thieves but what if you catch the “wrong” one that does carry a knife?
I would probably still do exactly the same when I would witness something like this again as it is something we do instinctively but I know out of experience that most Chinese would not do the same for you when they see something happening to you.
Not just with pickpocketing but also when there is a fight or traffic accident. A lot of them will watch but 99% will not intervene or help.
Unfortunately the “harmonious society” is more a good sounding propaganda slogan than an actual society characteristic…