Ask any cop who has had an abused woman attack him/her while they are arresting the hump who just split her lip and blackened her eye
I'm no cop, but many moons ago, just out of the Navy, I went to the aid of a young woman who was being viciously beaten and forced into a car. She was screaming "STOP!", "Someone HELP ME!". Her face and shirt were covered with blood.
I ran full speed into her attacker, leading with an elbow to the side of his head. He skidded across the hood of the car, falling unconsious on the other side.
The next thing I know, this woman smacks ME in the side of the head with small wooden bat, screaming "Leave my boyfriend alone!".
"Ouch! Fine. You don't have to live like that, but if you choose to, It's not my business."
Even after that, if I was in one of your scenarios, and I thought it likely that someone was going to be hurt by the thief, I think I would intervene.
If I thought that money would be taken and that would be the end, I wouldn't.
I know it would be impossible to know for sure what their intentions are, but I'd go with my gut.