It's impossible to know what I would do until it happens. These guys are usually practised and things happen so fast...
I am a very stubborn person and I am ferocious when I see abuse to an animal or child in front of me and have put myself in what might have been considered 'danger' by making my feelings known and getting involved in situations that really were 'none of my business' but then a) what's right is right and people shouldn't think they can get away with something because everyone will turn a blind eye or 'give in' and b) I can't seem to help myself.
I agree that letting go of the bag is the sanest and prob the best thing to do but knowing my past 'knee-jerk' behaviour I will prob scream so loud, kick so hard and pull the bag so hard I will break my darling bag and get sued by the robber for making him/her deaf and infertile.