People in black BMW did the robbery? Wow, not what you'd expect. Then again, maybe they wanted another one.
As someone who was mugged at knife point in Buenos Aires recently while wearing very plain clothes and not being flashy (though I had the camera in a pocket)... it can happen and it is fast and like a blur. The two robbers decided the best course of action would be to smash my girlfriend's head on the ground (by grabbing her hair and yanking her to the pavement) quickly so I would care more about her than them. They were right. I looked at the guys with the knives and decided they were going to do what they want anyway. So, I just went to her and made sure she was ok. They wanted her bag (again a non name brand black bag...not fancy at all). They got it because she said "just take it" and they ran. They didn't get anything from me, but I had the iPhone and the camera they most likely were looking for.
People who think that this cannot happen, I would say, yes, it can. People who think they would be tough enough to kick ass, maybe you would be. However, I don't carry a knife or gun or am I a career criminal / professional fighter / self proclaimed bad-ass used to dealing with this type of behavior. I haven't fought anyone in 20 years. So in this case, the best course of action was to give them what they wanted and let them get out of there. If not, I would have been stabbed for sure.
It was fast and my memory of the incident was basically 5 images... 1) the first guy with a knife distracting us 2) my girlfriend being brought to the ground by a guy behind us (second guy) 3) the second guy with a knife 4) my girlfriend on the ground, giving up her bag with 4 hands on the bag (a good sign that we weren't going to be stabbed) and 5) them running away with the bag. The cops caught one guy... the one without her bag of course. Yeah, it happens... even when being careful and trying to blend in. Of course I'm a gringo so I don't blend in well in BA. We came to find out that it was during a change-over in police patrolling that block and the two guys knew that. Wrong place, WRONG time.